This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by De Clercq, E. Search for Related Content PubMed PubMed Citation Articles by De Clercq, E.

Next Article 

Clinical Microbiology Reviews, October 2003, p. 569-596, Vol. 16, No. 4
0893-8512/03/$08.00+0     DOI: 10.1128/CMR.16.4.569-596.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Clinical Potential of the Acyclic Nucleoside Phosphonates Cidofovir, Adefovir, and Tenofovir in Treatment of DNA Virus and Retrovirus Infections

Rega Institute for Medical Research, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium

SUMMARY

INTRODUCTION

MECHANISM OF ACTION

ANTIVIRAL ACTIVITY SPECTRUM

ANIMAL MODEL INFECTIONS

Cidofovir

Adefovir

Tenofovir

CLINICAL USEFULNESS

Cidofovir

CMV retinitis.

CMV disease.

Herpesvirus infections other than CMV infections.

Adenovirus infections.

Polyomavirus infections.

HPV infections.

Poxvirus infections.

Adefovir

HIV infections.

HBV infections.

Tenofovir

HIV infections.

HBV infections.

RESISTANCE DEVELOPMENT

Cidofovir

Adefovir

Tenofovir

CONCLUSION

ACKNOWLEDGMENTS

REFERENCES

Top Next References

INTRODUCTION

Top Previous Next References

 
Most of the antiviral compounds that are currently used in the treatment of herpesvirus (herpes simplex virus (HSV), varicella-zoster virus (VZV), and cytomegalovirus (CMV)) infections (63) can be described as acyclic nucleoside analogues: acyclovir, penciclovir, and ganciclovir (Fig. 1). To increase their oral bioavailability, acyclovir, ganciclovir, and penciclovir have been converted to their oral prodrug forms (termed valaciclovir, valganciclovir, and famciclovir, respectively). Following their absorption from the gut, these compounds are reconverted to the parent compounds before reaching their target organ(s).

View larger version (25K): [in this window] [in a new window]   FIG. 1. Acyclic nucleoside analogues and acyclic nucleotide analogues (acyclic nucleoside phosphonates).

View larger version (19K): [in this window] [in a new window]   FIG. 2. Intracellular metabolism of acyclic nucleoside analogues and acyclic nucleotide analogues. The former need three phosphorylation steps whereas the latter only need two to be converted to their active metabolites (the dNTP analogues). Symbols: •, phosphate; , phosphonate.

Acyclic nucleoside phosphonates (61) possess a phosphonate group attached to the acyclic nucleoside moiety through a stable PC bond. In contrast to the phosphate group (which is attached through a POC bond), a phosphonate group (PC bound) cannot be cleaved off by cellular hydrolases (esterases). Foremost among the acyclic nucleoside phosphonates that have been pursued as antiviral agents are cidofovir (HPMPC) [(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine], adefovir (PMEA) [9-(2-phosphonylmethoxyethyl)adenine], and tenofovir (PMPA) [(R)-9-(2-phosphonylmethoxypropyl)adenine]. Because of their limited oral bioavailability, the last two compounds have been converted to their oral prodrug forms, adefovir dipivoxil or bis(pivaloyloxymethyl)-PMEA [bis(POM)-PMEA] and tenofovir disoproxil or bis(isopropyloxycarbonyloxymethyl)-PMPA [bis(POC)-PMPA], respectively (Fig. 1).

MECHANISM OF ACTION

Top Previous Next References

 
Since the acyclic nucleoside phosphonates already contain a phosphate-mimetic group, stably attached through a PC bond, they need only two, instead of three, phosphorylation steps to reach the active metabolite stage (Fig. 2). Thus, acyclic nucleoside phosphonates do not depend on the virus-induced kinase to exert their antiviral action, and, in "bypassing" the nucleoside kinase step, the acyclic nucleoside phosphonates may be expected to act against a broad range of DNA viruses, including hepadnaviruses (hepatitis B virus [HBV]) and retroviruses (human immunodeficiency virus [HIV]), i.e., all the viruses that use for their replication a DNA polymerase via which the active metabolites of the acyclic nucleoside phosphonates could enter into competition with the normal substrates (dNTPs).

Cidofovir (HPMPC) is phosphorylated by pyrimidine nucleoside monophosphate (PNMP) kinase to cidofovir monophosphate (HPMPCp), which is then further phosphorylated by nucleoside diphosphate (NDP) kinase, pyruvate kinase, or creatine kinase to cidofovir diphosphate (HPMPCpp) (50). Both phosphorylation steps can occur in both uninfected and virus-infected cells. For adefovir (PMEA), and presumably tenofovir (PMPA) as well, phosphorylation to the diphosphate form (PMEApp and PMPApp) can be achieved in one or two steps through the aid of 5-phosphoribosyl-1-pyrophosphate synthetase (14) or AMP (dAMP) kinase (139), respectively.

Cidofovir inhibits CMV replication at a 50% antivirally effective concentration of 0.1 µg/ml, which is 1,000-fold lower than the 50% cell-inhibitory concentration (IC₅₀) required to inhibit normal cell proliferation (199). This antiviral selectivity is also reflected at the viral DNA synthesis level, since cidofovir inhibits CMV DNA synthesis at a concentration (IC₅₀, 0.1 µg/ml) that is 1,000-fold lower than the concentration (IC₅₀, 100 µg/ml) required to inhibit cellular DNA synthesis (155). In contrast to ganciclovir, which provides only a weak and transient inhibition of viral DNA synthesis and viral replication, cidofovir was found to confer a pronounced and prolonged inhibition of viral DNA synthesis and viral replication, lasting for at least 7 days after an exposure time as short as 6 h postinfection (156).

The long-lasting antiviral action of cidofovir allows infrequent dosing with the drug (i.e., only once a week or every other week), which clearly distinguishes cidofovir from other antiviral drugs (acyclovir, penciclovir, and ganciclovir), which have to be administered several times daily to sustain an antiviral response. The long-lasting antiviral action of cidofovir can be attributed to the long half-life of the HPMPC metabolites (HPMPCp, HPMPCpp, and HPMPCp-choline) that are formed intracellularly following uptake of HPMPC by the cells (presumably by endocytosis) (53). In particular, HPMPCp-choline may serve as the intracellular depot form of HPMPC, since its intracellular half-life as extremely long (48 h) (49, 100).

The cellular uptake of HPMPC is rather slow, due to the presence of the negatively charged phosphonate group. This uptake can be greatly enhanced (2) if the phosphonate group is esterified, as in 1-O-hexadecyloxypropyl-cidofovir (HDP-CDV); compared to cidofovir itself, HDP-CDV demonstrated a multiple-log enhancement in antiviral activity against both poxviruses (vaccinia virus and cowpox virus) (110) and herpesviruses (HSV and CMV) (18).

The acyclic nucleoside phosphonates owe their selective antiviral activity to the fact that in their diphosphorylated form, they have a higher affinity (lower K_i value) for the viral DNA polymerases (HSV-1 DNA polymerase, CMV DNA polymerase, and HIV-1 reverse transcriptase [RT]) than for cellular DNA polymerases , ß, , , and (references 116 and 147 and references therein). HPMPCpp, PMEApp, and PMPApp interact as competitive inhibitors or alternate substrates with the normal substrates (i.e., HPMPCpp with dCTP and PMEApp and PMPApp with dATP) for the viral DNA polymerases. The incorporation of one molecule of PMEA or PMPA at the 3' end of the growing DNA chain suffices to terminate further chain elongation (147). PMPA is more faithful a chain terminator of the HIV-1 RT reaction than PMEA, since it is incorporated to a lower extent by the cellular DNA polymerases , ß, and (51). The 3' 5' exonuclease activity of DNA polymerases and have proved to be able to excise PMEA from the 3'-OH end of DNA, albeit at a rate 1 order of magnitude lower than that of the dAMP residue (24). PMPApp was found to be an exceptionally poor substrate (1,000-fold less efficient than dATP) and a weak inhibitor of cellular DNA polymerases , , and (25). Thus, PMPApp may minimally interfere with nuclear DNA synthesis, and this may at least partially explain its low cytotoxicity and favorable safety profile in the treatment of HIV infections.

For HPMPC, two consecutive incorporations (at the 3' end of the DNA chain) are required to efficiently shut off CMV DNA elongation (226). HPMPC is incorporated in the DNA product with the correct complementation to dGMP in the template, and the incorporated HPMPC is not excised by CMV DNA polymerase. Incorporation of one HPMPC molecule causes a marked decrease in the rate of DNA elongation; incorporation of two consecutive HPMPC molecules causes a virtual stop in DNA synthesis, so that incorporation of a third (consecutive) HPMPC molecule cannot be detected. Incorporation of two HPMPC molecules separated by a dNMP (dAMP, dGMP, or dTMP) also drastically decreases the rate of DNA chain elongation by CMV DNA polymerase (226).

While the antiviral effects of cidofovir can be attributed to the interaction of HPMPCpp with the viral DNA polymerase and incorporation of HPMPC into the viral DNA chain, its specific inhibitory activity against the proliferation of human papillomavirus (HPV)-infected cells (6) must imply additional or alternative mechanisms of action. The antiproliferative effect of cidofovir on the growth of HPV-infected cells, akin to its inhibitory effect on the growth of nasopharyngeal carcinoma (142, 154), may be ascribed to the induction of apoptosis (4, 7). This, in turn, may be related to the ability of cidofovir to restore the function of the tumor suppressor proteins p53 and pRb (which are neutralized by the oncoproteins E6 and E7, respectively) in HPV-infected cells (1).

Andrei et al. (4) have reported that in HPV-infected, but not uninfected, human keratinocytes, cidofovir caused cell death by apoptosis, as evidenced by several parameters of apoptosis: (i) induction of caspase protease activity, (ii) translocation of phosphatidylserine from the inner part of the plasma membrane to the outer layer, (iii) disintegration of the nuclear matrix protein, (iv) DNA fragmentation, and (v) number of cells in apoptotic phase following cell cycle analysis. Induction of apoptosis in HPV-positive cells by cidofovir was associated with accumulation of the tumor suppressor protein p53 and the cyclin-dependent kinase inhibitor p21/WAF-1 (4).

Polyomaviruses are genomically and functionally related to papillomaviruses; like papillomaviruses, polyomaviruses are able to induce tumors, e.g., hemangiomas in rats (127). In fact, cidofovir can successfully suppress polyomavirus-induced tumor formation in rats (127). It is therefore tempting to speculate that in its action against polyomavirus- associated lesions, cidofovir follows the same strategy as against HPV-associated lesions, namely, induction of apoptosis.

ANTIVIRAL ACTIVITY SPECTRUM

Top Previous Next References

 
The acyclic nucleoside phosphonates possess an antiviral activity spectrum that is quite different from that of the "classical" nucleoside analogues acyclovir, ganciclovir, and penciclovir. Whereas the activity spectrum of acyclovir, ganciclovir, and penciclovir is restricted to the main herpesviruses HSV, VZV, CMV, and Epstein-Barr virus (EBV) and only strains that are either TK⁺ (HSV, VZV, and EBV) or PK⁺ (CMV), cidofovir (62, 64) is active against all herpesviruses (including EBV and its murine counterpart, murine herpesvirus 68 [MHV-68] [151], human herpesvirus 6 [HHV-6], HHV-7, and HHV8, the putative cause of Kaposi's sarcoma [kS] [109, 138], and the TK^- HSV, TK^- VZV, and PK^- CMV strains); furthermore, cidofovir is also active against adenovirus, polyomavirus, papillomavirus, and poxvirus (Table 1). In particular, the activity of cidofovir against poxviruses (including vaccinia virus, cowpox virus, camelpox virus, monkeypox virus, and variola virus [192]) and parapoxviruses (such as orf virus [148]) has recently received considerable attention (66-68). The activity of cidofovir against molluscum contagiosum virus (MCV) is surmised from recent observations that cidofovir in HIV-infected patients effected a complete resolution of recalcitrant molluscum contagiosum lesions following either topical or systemic administration (59, 103, 137, 212, 231). The comparative activities of cidofovir and various other compounds against HHV-6, HHV-7, HHV-8, and (murine and primate) polyomaviruses (8, 150, 173, 205) have been reviewed recently (70).

ANIMAL MODEL INFECTIONS

Top Previous Next References

 

In particular, the efficacy of topical cidofovir against adenovirus infections has been demonstrated in the rabbit ocular model (79, 175, 177). In the rabbit model, topical cidofovir proved significantly more efficacious against HSV-1 keratitis than did topical acyclovir or trifluridine (176). Cidofovir and trifluridine were both highly effective in preventing the development of experimental HSV-1 stromal disease in rabbits (108).

Wherever cidofovir was directly compared for its antiviral efficacy with other antiviral drugs (acyclovir or ganciclovir), it proved clearly superior; e.g., to acyclovir, in the treatment of mice infected intracerebrally with HSV (69, 227) and to ganciclovir in the treatment of both immunocompetent and immunodeficient mice infected with murine CMV (152, 153, 187). In the treatment of acute murine CMV-induced myocarditis in mice, with treatment beginning 24 h postinfection, cidofovir was also clearly more efficacious than ganciclovir (125).

In a lethal immunosuppression model in BALB/c mice, cidofovir (at 5 mg/kg/day) provided significant protection (i.e., 13-day delay in death and 3,000-fold reduction in virus titers in the lungs) against MHV-68, a rodent virus related to EBV (191); it was concluded that cidofovir may be an excellent candidate for treating EBV infections in humans, particularly in immunocompromised patients.

In the cottontail rabbit papillomavirus model, complete cures were obtained with topical 1% cidofovir at dosing schedules of twice daily for 8 weeks beginning at 4 weeks after infection, when papillomas were clearly visible (44, 81). If cidofovir treatment was combined with viral DNA vaccination, recurrence rates could be further reduced (45).

Cidofovir has proven to be efficacious in the treatment of lethal (systemic) vaccinia virus infections in mice with severe combined immune deficiency (SCID mice) (149), as well as in the treatment of lethal vaccinia virus respiratory infections in BALB/c mice (188, 190), where a single dose of the compound (given subcutaneously 1 day postinfection) sufficed to reduce mortality from 100 to 0%.

A single subcutaneous or intranasal (aerosolized) dose of cidofovir, administered 1 day before or after an intranasal (aerosolized) cowpox virus infection, resulted in 100% survival of the infected mice (33, 34, 189), which means that aerosolized cidofovir could be effective in the prophylaxis or early postexposure therapy of human smallpox or monkeypox virus infection. Given the enhanced oral bioavailability of HDP-CDV compared to cidofovir itself (J. W. Huggins, R. O. Baker, J. R. Beadle, and K. Y. Hostetler, Abstr. 15th Int. Conf. Antiviral Res., abstr. 104, 2002; K. L. Winegarden, S. L. Ciesla, K. A. Aldern, J. R. Beadle, and K. Y. Hostetler, Abstr. 15th Int. Conf. Antiviral Res., abstr. 105, 2002), oral dosing of HDP-CDV may be a viable alternative to intravenous or aerosol delivery of cidofovir to human lungs in patients with a respiratory poxvirus infection or DNA virus infections in general.

Cidofovir has demonstrated remarkable activity as an antitumor agent in a number of animal models of tumor growth: (i) nasopharyngeal carcinoma xenografts in athymic nude mice (142, 154), (ii) human cervical carcinoma xenografts in athymic nude mice (7), (iii) polyomavirus-induced hemangiomas in rats (127), (iv) hemangiosarcoma development in nude mice (126), (v) fibroblast growth factor 2-induced vascular tumor formation in nude mice and SCID mice (128), and (vi) murine melanoma B16 in C57B16/J mice (172). While the mechanism of action of cidofovir against melanoma cell growth was not resolved (172), its inhibitory effect on the growth of hemangiosarcoma (124), endothelium-derived tumors (128), and nasopharyngeal carcinoma (142, 154) could be unequivocally attributed to the induction of apoptosis.

Oral adefovir dipivoxil was also shown to significantly reduce the viral load in the liver as well as in the serum of transgenic mice expressing HBV: the minimum effective dose was less than 0.1 mg/kg/day for the serum HBV load (106). When evaluated under the same conditions, oral lamivudine at doses up to 500 mg/kg/day only marginally reduced the level of HBV DNA in serum and did not significantly reduce the HBV DNA level in the liver (106).

Adefovir has also proven to be more effective than lamivudine in suppressing viremia and intrahepatic viral DNA in ducklings experimentally infected with duck hepatitis B virus. Although adefovir effected a potent reduction of viremia, it was unable to prevent the initial formation of covalently closed circular DNA (cccDNA). Nor was lamivudine able to do so (74).

Follow-up studies ascertained that postexposure prophylaxis with tenofovir, as well as adefovir, can provide long-term protection against subsequent heterologous SIV challenge, and this was attributed to activation of the antiviral immune response (215). In another study, tenofovir treatment, started 7 days after inoculation of rhesus macaques with SHIV, reduced plasma viral RNA levels to undetectable, with parallel decreases in the infectivity of plasma and infectious cells in peripheral blood mononuclear cells and cerebrospinal fluid (CSF) and stabilization of CD4⁺ T-cell numbers; following cessation of treatment after 12 weeks, the CD4⁺ T-cell counts normalized and stabilized in the normal range, despite persistent low-level infection (194).

In newborn rhesus macaques inoculated orally with the highly virulent SIV_mac251 strain within 3 days of birth, tenofovir treatment from 5 days old for either 14 or 60 days caused reduced virus levels and enhanced antiviral immune responses (220). If the newborn macaques, inoculated orally with SIV_mac251 at the age of 3 days, received a subcutaneous injection of tenofovir (4 mg/kg) either 4 h before or 20 h after, or 1 and 25 h after infection, or a single dose of tenofovir (30 mg/kg) at 1 h after SIV inoculation, they remained SIV negative and seronegative (218).

The use of tenofovir has also been investigated in the postexposure prophylaxis of intravaginal infection of pig-tailed macaques with HIV-2 (159). Tenofovir was administered subcutaneously at 30 mg/kg for 28 days, starting at 12, 36, or 72 h after viral inoculation: early intervention (i.e., treatment started at either 12 or 36 h after virus inoculation) completely abrogated HIV infection via intravaginal exposure (159).

CLINICAL USEFULNESS

Top Previous Next References

 

View larger version (20K): [in this window] [in a new window]   FIG. 3. Time to progression of CMV retinitis: immediate treatment with cidofovir versus deferred treatment with cidofovir. Cidofovir was given intravenously at 5 mg/kg once weekly for 2 weeks followed by 5 mg/kg every other week. Administration of cidofovir was accompanied by intravenous hydration with 2 liters of normal saline and 4 g of probenecid orally (2 g at 3 h before, 1 g at 2 h after, and 1 g at 8 h after each cidofovir infusion). (Reprinted from reference 119 with permission of the publisher.)

CMV retinitis. In the study reported by Lalezari et al. (119), cidofovir was found to be efficacious in delaying the progression of CMV retinitis (in patients with AIDS) when given intravenously at 5 mg/kg once weekly for 2 weeks (induction therapy) followed by once every other week (maintenance therapy). Treatment was associated with manageable side effects. Strict adherence to monitoring of renal function before cidofovir was administered and concomitant administration of probenecid and saline hydration appeared to minimize drug-related nephrotoxicity (119). In the SOCA/ACTG trial (209a), cidofovir at two treatment regimens (induction therapy, as indicated above; maintenance therapy at 5 mg/kg every other week [high dose] or 3 mg/kg every other week [low dose]) effectively slowed the progression of CMV retinitis (in patients with AIDS), with the high-dose regimen being clearly more efficacious than the low-dose regimen (209a). Further follow-up over a long-term period (210) showed similar rates of progression and median times to progression with both low maintenance and high maintenance doses of cidofovir to those reported in the initial study (210). From the long-term follow-up study (21), it was concluded that cidofovir is effective for the treatment of CMV retinitis but that it has potential for toxicity (both renal and ocular), which typically resolves on discontinuation of therapy.

The efficacy of cidofovir therapy for AIDS-associated CMV retinitis has been confirmed in patients receiving highly active antiretroviral therapy (HAART), although in this study a higher incidence of iritis was noted, probably as a consequence of the patients' enhanced ability to mount an inflammatory response (21). In fact, anterior uveitis occurs after a mean of 8.5 intravenous infusions of cidofovir (52); it responds to treatment with topical corticosteroids and mydriatics and does not preclude the continuation of cidofovir therapy (3) unless ocular hypotony develops (13).

To assess the effect of intravenous cidofovir on delaying the progression of previously treated, relapsing CMV retinitis, a randomized, controlled comparison of two maintenance doses of cidofovir was conducted: AIDS patients with CMV retinitis that had progressed despite treatment with ganciclovir, foscarnet, or both, were randomized to receive induction cidofovir (5 mg/kg once weekly for 2 weeks) and then maintenance therapy with either 5 mg/kg or 3 mg/kg once every other week (118). The median time to retinitis progression as assessed by retinal photography was not reached in the 5-mg/kg dose group and was 49 days in the 3-mg/kg dose group (118).

Recent studies have indicated that the regimens of, on the one hand, ganciclovir implantation (i.e., a surgically placed intraocular implant) plus oral ganciclovir and, on the other hand, intravenous cidofovir are equally effective in slowing the progression of CMV retinitis (Fig. 4) and preventing visual loss (Fig. 5) (211). A phaseI study indicated that combination therapy of intravenous cidofovir (5 mg/kg intravenously every 2 weeks) with oral ganciclovir (1 g orally three times a day) may further enhance clinical efficacy (104).

View larger version (11K): [in this window] [in a new window]   FIG. 4. Kaplan-Meier curves showing the probability of retinitis progression in AIDS patients with CMV retinitis who were randomized to a regimen of ganciclovir implant plus oral ganciclovir (Gcv) or intravenous cidofovir (Cdv). Cidofóvir was administered according to the dosage schedule explained in the legend to Fig. 3. Ganciclovir was administered as a surgical intraocular implant together with an oral dosage regimen of 1 g three times daily. (Reprinted from reference 211 with permission of the publisher.)

View larger version (11K): [in this window] [in a new window]   FIG. 5. Kaplan-Meier curves showing the probability of a 15-letter or greater loss of visual acuity in AIDS patients with CMV retinitis who were randomized to a regimen of ganciclovir implant plus oral ganciclovir (Gcv) or intravenous cidofovir (Cdv). Treatment and dosage regimens were as explained in the legends to Fig. 3 and 4. (Reprinted from reference 211 with permission of the publisher.)

CMV disease. In clinical trials cidofovir has proven efficacious not only against CMV retinitis but also against other manifestations of CMV disease, e.g., intravenously against asymptomatic CMV infection in HIV-infected patients (121, 165). Cidofovir can be recommended as a preemptive treatment for CMV disease (i.e., treatment initiated if CMV antigen or DNA is detected in the blood) after allogeneic blood stem cell transplantation (163). In the study reported by Platzbecker et al. (163), 9 (90%) of 10 patients showed a response to cidofovir treatment, with 7 of the 9 experiencing a complete clearance of the virus (pp65 negative, PCR negative); treatment-related toxicity was moderate, with 4 patients developing reversible renal impairment.

In another study (129), half of the patients who were treated for CMV disease after allogeneic stem cell transplantation responded to cidofovir therapy, as did 66% of the patients who had failed or relapsed after previous preemptive therapy with ganciclovir or foscarnet and 62% of the patients in whom cidofovir was used as the primary preemptive therapy (129). Similar results were obtained in another prospective study of primary preemptive therapy with cidofovir (31).

Cidofovir should also be considered as second-line therapy in patients with CMV disease failing to respond to ganciclovir or foscarnet (54, 129). According to a case report (28), cidofovir effected a complete resolution of CMV retinitis, CMV encephalitis, and CMV esophagitis after only 2 months of intravenous therapy and after initial attempts to stop the disease with ganciclovir and foscarnet had been unsuccessful.

Herpesvirus infections other than CMV infections. Cidofovir has proven efficacious not only against CMV disease but also against HSV infections, e.g., when injected intravenously (115, 122) or applied topically (120, 195) to treat acyclovir-resistant mucocutaneous HSV infections. Such acyclovir-resistant HSV infections may be quite severe in immunosuppressed (e.g., AIDS) patients. As described in a recent case report, a severe perianal HSV-2 infection in an AIDS patient that was refractory to both ganciclovir and acyclovir treatment healed virtually completely within 30 days of treatment with intravenous cidofovir at a dose of 5 mg/kg once weekly for a total of 3 weeks (115). In a child with AIDS presenting with a facial HSV ulcer that had developed resistance to acyclovir and did not respond to foscavir either, local application of cidofovir 1% led to a prompt recovery (123).

The safety and efficacy of single topical applications of 1, 3, and 5% cidofovir gel have been monitored in a multicenter double-blind, randomized phase I/II dose-escalating study for the treatment of early, lesional, recurrent genital herpes at five Canadian outpatient sites (178). Cidofovir gel at all strengths significantly decreased the median time to negative virus culture: 3.0 days in the placebo group versus 2.2, 1.3, and 1.1 days in the 1, 3, and 5% cidofovir gel treatment groups, respectively (Fig. 6). Application site reactions occurred in 3, 5, 19, and 22% of the patients in these four groups, respectively. Additional studies are warranted to further identify the optimal efficacious dose of cidofovir that can be tolerated (178).

View larger version (18K): [in this window] [in a new window]   FIG. 6. Cumulative proportion of patients with genital herpes who converted to HSV culture negativity following a single topical application of placebo, or 1, 3, or 5% cidofovir gel. (Reprinted from reference 178 with permission.)

There is anecdotal evidence for the efficacy of cidofovir in the treatment of oral hairy leukoplakia (which is presumably due to EBV infection) (121). According to a case report, treatment with cidofovir together with rituximab (anti-CD20 monoclonal antibody) led to complete remission of an EBV-associated lymphoma involving the central nervous system (95). Also, in another case of EBV-associated lymphoma, intralesional injections of cidofovir led to a marked regression of the lesions (A. Meerbach, M. Schacke, P. Hyckel, H. Kosmehl, and P. Wutzler, Abstr. 15th Int. Conf. Antiviral Res., abstr. 91, 2002). Cidofovir may also be expected to be effective in the adjunctive treatment of nasopharyngeal carcinoma since it strongly inhibits the growth of nasopharyngeal carcinoma xenografts in nude mice, apparently through the induction of apoptosis (142, 154).

Whether cidofovir would be effective in the treatment of KS máy depend on the interplay of different factors. In two patients with AIDS, an important regression of all cutaneous KS lesions was noted after 3 months of cidofovir treatment (5 mg/kg intravenously at 1-week intervals for the first two injections and every 2 weeks thereafter) (136). A clinical response of KS lesions to intravenous cidofovir treatment has also been noted in an HIV-negative homosexual man (84). In another patient without AIDS, classical KS was not affected by intralesional injections of cidofovir (186). Successful treatment of KS in a HIV-negative man was observed with intravenous cidofovir treatment added to liposomal daunorubicin (12). Abatement of cutaneous KS was noted in a patient with AIDS and CMV retinitis who was treated with cidofovir (94). Since opportunistic infections such as those due to CMV may trigger the development of KS through the release of cytokines and growth factor, inhibition of CMV replication by cidofovir may contribute to the abatement of KS lesions (185).

Adenovirus infections. Adenovirus infection can be particularly severe in allogeneic stem cell transplant recipients. Intravenous cidofovir (with concomitant probenecid) at a dose of 5 mg/kg/week for 2 weeks and then every 2 weeks for a total of five doses proved successful in suppressing the manifestations of adenovirus disease in an allogeneic stem cell transplant recipient (174).

In five children who had developed a systemic adenovirus infection after bone marrow transplantation for leukemia, cidofovir (administered intravenously at 5 mg/kg once weekly for 3 weeks, then every 2 weeks) effected clinical improvement of diarrhea, cystitis, and fever and a concomitant disappearance of the virus, as assessed by PCR and cell culture (124).

From a retrospective analysis of 35 patients who were identified with adenovirus infection after having undergone allogeneic hematopoietic stem cell transplantation, it appeared that only cidofovir and donor leukocyte infusion were effective options whereas ribavirin and vidarabine were not (30).

In a prospective trial initiated to evaluate cidofovir in the treatment of adenovirus infections in hematopoietic stem cell transplant recipients, eight patients were enrolled on a dosage schedule of 1 mg/kg (intravenously) three times weekly (101). All of these patients eventually achieved long-term viral suppression and clinical improvement, although six patients needed prolonged cidofovir therapy for up to 8 months before the cidofovir administration could be stopped without adenovirus recurrence; no dose-limiting nephrotoxicity was observed, and discontinuation of the drug was not required in any patients (101).

Although topical cidofovir has proven to be effective against adenovirus and HSV keratoconjunctivitis in animal models (79, 108, 175-177), it has not been intensively pursued for the corresponding indications in humans. There is an anecdotal report of the potential effectiveness of 0.2% cidofovir eye drops in a patient with adenoviral conjunctivitis (91), but a larger study failed to show a statistically significant effect of topical 0.2% cidofovir on the course of the acute phase of adenoviral conjunctivitis (98). Eye drops of 1% cidofovir did prove effective in the prevention of severe corneal opacities and, concomitantly, caused inflammation of the eyelids and conjunctiva (99). This local toxicity is probably related to the too frequent administration (4 or 10 times daily) of the 1% cidofovir eye drops; further clinical studies are needed to delineate the optimal treatment regimen for adenovirus keratoconjunctivitis.

Polyomavirus infections. There are a vast number of reports pointing to the efficacy of cidofovir (given at the dosage regimen recommended for the treatment of CMV retinitis in AIDS patients) in the treatment of PML in AIDS patients. For a series of 12 patients with AIDS-associated PML, a significant correlation was found between JC virus load and survival time (206). In one case, the JC virus load in CSF, which can be considered a prognostic parameter for the clinical outcome of PML, decreased and then became undetectable after cidofovir treatment; this was associated with clinical improvement (206). This patient had been receiving HAART for 2 months without experiencing a modification in JC virus load, before the cidofovir therapy was started (206).

In HIV-infected hosts, the clinical course of PML is almost invariably fatal and there is no proven effective therapy for this condition. Initial anecdotal reports of responses to cytarabine (cytosine arabinoside) have not been confirmed in larger or randomized trials (78). If, however, cytarabine was combined with cidofovir, a remarkable turnaround of PML was noted in a patient with advanced HIV disease (27); cidofovir may well have been the major factor contributing to the successful resolution of PML in this patient (27).

In a multicenter study of consecutive HIV-positive patients with histologically or virologically proven PML, 26 patients were treated with HAART only while 14 patients were treated with HAART plus cidofovir (5 mg/kg intravenously per week for the first 2 weeks and alternate weeks thereafter) (77). Cidofovir added to HAART was associated with a more effective control of JC virus replication, with improved neurological outcome and survival compared with that due to HAART alone (Fig. 7) (77).

View larger version (17K): [in this window] [in a new window]   FIG. 7. Efficacy of cidofovir combined with HAART in AIDS-associated PML. Kaplan-Meier curves show the survival of patients receiving HAART plus cidofovir (continuous line) versus patients receiving HAART only (dashed line) (log rank, P = 0.01). Cidofovir was administered intravenously at 5 mg/kg once weekly for the first 2 weeks and on alternate weeks thereafter (see also the legend to Fig. 3). (Reprinted from reference 77 with permission of the publisher.)

View larger version (98K): [in this window] [in a new window]   FIG. 8. Response of PML to cidofovir after failure of HAART alone. T2-weighted axial brain magnetic resonance imaging scan shows marked neuroradiologic improvement 12 weeks after addition of cidofovir to HAART. Cidofovir was administered intravenously at the dosage schedule explained in the legend to Fig. 3. (Reprinted from reference 75 with permission of the publisher.)

In a follow-up study of the multicenter observational study mentioned above (77), it was ascertained that the 1-year cumulative probability of survival was 0.61 with cidofovir and 0.29 without. After adjusting for baseline CD4 counts, the JC virus load in the CSF, Karnofsky score, and use of HAART prior to the onset of PML, the use of cidofovir was independently associated with a reduced risk of death (76).

In another monocenter observational study of the effect of cidofovir on AIDS-associated PML, the 1-year cumulative probability of being active was 62% in the cidofovir-plus-HAART group, compared to 53% in the HAART group (87). An additional benefit with respect to survival was observed in patients who were given cidofovir after adjustment to the baseline variables CSF JC virus load, CD4 cell count, and expanded disability status scale (87).

Although there is overwhelming evidence to suggest that cidofovir offers at least partial benefit in the treatment of JC virus-associated PML (see above), its role in the treatment of BK polyomavirus-associated hemorrhagic cystitis is less well established. In a patient with BK polyomavirus-associated acute hemorrhagic cystitis who had received an allogeneic bone marrow transplant, cidofovir proved efficacious (90), but in another case of BK virus-associated hemorrhagic cystitis in an HIV-infected patient, there was apparently no response to cidofovir (17). BK virus-associated nephropathy has been increasingly recognized as an important cause of renal transplant dysfunction, and cidofovir (at a dose as low as 0.25 to 1 mg/kg) was found to reduce BK viruria to undetectable levels, accompanied by stable renal function for 6 to 26 months post-therapy.

HPV infections. Complete and permanent remissions of papillomatous lesions have been achieved following either topical gel application or direct intralesional injections of cidofovir. The first case of a hypopharyngeal/esophageal papilloma (due to HPV-16) that showed complete regression after topical cidofovir therapy was reported several years ago (Fig. 9) (216).

View larger version (87K): [in this window] [in a new window]   FIG. 9. Hypopharyngeal papilloma before and after treatment with cidofovir. Cidofovir was administered by local injection (directly into the tumor) at 1.25 mg/kg at weekly intervals. Complete regression of the tumor was achieved after seven injections (216).

In another double-blind, placebo-controlled, study of 1% cidofovir gel in the treatment of anogenital HPV infections, a partial to complete response was observed in 84.2% of the cidofovir-treated patients compared to 18.2% of the placebo-treated patients (197).

An open randomized prospective study indicated that topical 1% cidofovir gel was more effective than electrocautery in preventing recurrences of genital warts in HIV-infected patients (relapse rates, 35.29 and 73.68%, respectively) and that the two procedures, if combined, effected a complete response (158).

Initial clinical trials with 1% cidofovir topical gel also point to its efficacy in the treatment of cervical intraepithelial neoplasia grade III (196): partial to complete responses were observed histologically in the majority of the patients and were confirmed by PCR, and this effect was seen after only three applications (every other day) (196). Complete and permanent eradication of vulvar intraepithelial neoplasia grade III was achieved with topical 1% cidofovir in a patient who did not respond to interferon and isotretinoin (114). Also, bowenoid papulosis (perianal intraepithelial neoplasia) in an AIDS patient was found to respond to cidofovir: after three treatment cycles (0.4% cidofovir cream twice daily for five consecutive days every 15 weeks), the lesions completely disappeared (80). A similar, complete cure was seen in an AIDS patient with bowenoid papulosis of the penis after three rounds of 1% cidofovir cream applications (once daily for 5 days) (Fig. 10) (201).

View larger version (50K): [in this window] [in a new window]   FIG. 10. Topical cidofovir in the treatment of Bowenoid papulosis of the penis. Cidofovir was administered topically at 1% in Beeler base. Two months after initiation of treatment (three courses of once-daily applications for 5 days), the lesion had completely disappeared (201).

View larger version (62K): [in this window] [in a new window]   FIG. 11. Laryngeal papilloma before and after treatment with cidofovir. Cidofovir (2.5 mg/ml) was injected directly into the tumor. Complete regression of the tumor was achieved after 15 injections over 9 months (200).

View larger version (74K): [in this window] [in a new window]   FIG. 12. Intralesional cidofovir for recurrent respiratory papillomatosis in children. Cidofovir (2.5 mg/ml) was injected into the tumor. Complete regression of the tumor was achieved after 15 injections every 2 to 3 weeks. (Reprinted from reference 167 with permission of the publisher.)

Cidofovir has also proved successful in the topical treatment of verruca vulgaris: two patients with a 2- to 3-year history of recurrent verruca vulgaris that did not respond to repeated conventional therapies (electrodesiccation, liquid nitrogen, cryosurgery, CO₂ laser ablation, etc.) experienced complete disappearance of the verruca vulgaris lesions following once- or twice-daily therapy with 3% topical cidofovir (229). These patients have remained completely free of lesions for more than 40 weeks (230). A large plantar wart (caused by HPV-66) in an AIDS patient completely resolved within 3 to 4 weeks following two courses of topical 3% cidofovir therapy (Fig. 13) (60).

View larger version (47K): [in this window] [in a new window]   FIG. 13. Topical cidofovir in the treatment of a large plantar wart (caused by HPV-66). Cidofovir was administered topically at 3% in emollient cream twice daily. The wart disappeared within 3 to 4 weeks. (Reprinted from reference 60 with permission of the publisher.)

In view of the circumstantial evidence for the effectiveness of cidofovir in the treatment of virtually all HPV-associated diseases, it is intriguing that in one particular patient with epidermodysplasia verruciformis, topical and systemic treatment of cidofovir had no apparent effect (169). On the other hand, cidofovir injected intralesionally proved successful in the treatment of squamous cell carcinoma, a disease that is supposedly not related to HPV (37). This antineoplastic activity may be based on the same mechanism, i.e., induction of apoptosis, that underlies the inhibitory effect of cidofovir on the growth of HPV-associated tumors.

Poxvirus infections. Cidofovir is effective in the treatment of molluscum contagiosum, a cutaneous skin growth caused by the poxvirus MCV. Recalcitrant molluscum contagiosum in three AIDS patients resolved completely and permanently after either topical cidofovir treatment (3% cream, applied once daily) or intravenous cidofovir treatment (5 mg/kg each week for 2 weeks, followed by 5 mg/kg once every 2 weeks) (137). A similar complete and permanent remission of recalcitrant molluscum contagiosum was seen in an AIDS patient following nine cycles of intravenous cidofovir treatment (5 mg/kg once every 2 weeks) (Fig. 14) (103). Similar complete and durable resolution of molluscum contagiosum lesions was noted in children following topical treatment with cidofovir (at either 1% or 3%), e.g., in a boy with Wiskott-Aldrich syndrome (59), in two otherwise healthy children (231), and in two HIV-infected children (212).

View larger version (66K): [in this window] [in a new window]   FIG. 14. Intravenous cidofovir therapy in the treatment of molluscum contagiosum in AIDS patients. Cidofovir was administered intravenously (at 5 mg/kg once per week followed by 5 mg/kg once every 2 weeks); after nine cycles (4 months), the lesions had disappeared. (Reprinted from reference 103 with permission of the publisher.)

View larger version (66K): [in this window] [in a new window]   FIG. 15. Orf (ecthyma contagiosum) before and after treatment with cidofovir. Cidofovir was administered topically as a 1% cream in Beeler base once daily for repeated courses (5-days-on/5-days-off therapy), with complete resolution of the lesions after seven courses (2 to 3 months) (88).

Should smallpox vaccination, based on the use of the live vaccinia virus vaccine, be reinstalled, it would be formally contraindicated to use this vaccine in immunocompromised patients, whatever the cause of their immunodeficiency (primary immune deficiency, HIV infection, immunosuppressive therapy, etc.). Inadvertent use of the live vaccinia virus vaccine in such patients may lead to a serious, life-threatening, disseminated, and progressive vaccinia (111, 171). The therapy or prophylaxis of such complications may well represent a primary indication for the use of cidofovir.

Single-dose pharmacokinetics and the safety of oral adefovir dipivoxil have also been established in children infected with HIV-1 (102), and the results of this study were quoted as providing sufficient information to warrant proceeding to a multidose phase II study to further evaluate the safety, pharmacokinetics, and efficacy of adefovir dipivoxil in children. Hughes et al. (102) noted that in children the oral clearance (CL/F) of adefovir (dipivoxil) was greater than in adults (when a dose of 3.0 mg/kg in children was compared with a 60-mg dose in adults).

In one 24-week, randomized, double-blind, placebo-controlled multicenter study, adefovir dipivoxil, administered as a single daily oral dose of 120 mg added to stable antiretroviral therapy, effected a 0.4-log₁₀/ml decline from baseline in the HIV RNA plasma load, compared with no change in the placebo group (Fig. 16) (107). This reduction in viral load extended beyond 24 weeks but was accompanied by (reversible) nephrotoxicity (107). In another randomized, double-blind, placebo-controlled multicenter trial, no virologic or immunologic benefit was observed when oral adefovir dipivoxil (120 mg once daily) was added to background antiretroviral therapy in treating advanced HIV disease (85). The difference in the virologic results between the two studies (85, 107) may be explained by the fact that the patients in the latter study (85) had more advanced disease (lower CD4⁺ cell counts) and were more drug experienced than those in the former study (107). In the latter study (85), the use of adefovir dipivoxil (120 mg daily) was associated with considerable nephrotoxicity.

View larger version (26K): [in this window] [in a new window]   FIG. 16. Changes from baseline in HIV-1 RNA levels in patients is a 24-week, randomized, double-blind, placebo-controlled multicenter study. Patients with HIV-1 infection were randomized to receive either a single daily dose of 120 mg of adefovir dipivoxil or placebo for 24 weeks. Open-label adefovir was offered after 24 weeks. (Reprinted from reference 107 with permission of the publisher.)

Combination therapy with adefovir dipivoxil plus efavirenz, in highly treatment-experienced patients, resulted in a marked viral load suppression (>2.0 log₁₀ units at 24 weeks) but only in patients who had no prior experience with nonnucleoside RT inhibitors (184).

In view of the rather modest reduction in viral load observed with adefovir dipivoxil (only a 0.4-log₁₀/ml decline in viral load [Fig. 16] [107]) and the accompanying risk for nephrotoxicity (development of a Fanconi-like renal syndrome consisting of acidosis, proteinuria, glucosuria, hypophosphatemia, and elevated creatinine levels at the dosage used [120 mg once daily]), adefovir dipivoxil has not further been pursued for the treatment of HIV infections.

HBV infections. Adefovir dipivoxil has been actively pursued for the therapy of chronic hepatitis B. In a placebo-controlled phase I/II study, adefovir dipivoxil given orally at 125 mg as a single daily dose for 28 days effected a 1.8-log₁₀ pg/ml reduction in HBV DNA levels, compared to an increase of 0.01 log₁₀ pg/ml in the control patients with chronic HBV infection (89).

Lamivudine, the current drug of choice for the treatment of chronic hepatitis B, has been shown to elicit resistance in 20, 38, 49, 66, and 69 of patients after 1, 2, 3, 4, or 5 years of lamivudine therapy, respectively (C. E. Westland, C. S. Gibbs, M. D. Miller, M. Sullivan, J. Fry, C. L. Brosgart, M. Wulfsohn, and S. Xiong, Oral Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Diseases, abstr. 568, 2002). A 48-week, randomized, double-blind, placebo-controlled, multicenter study was designed to evaluate the efficacy of adefovir dipivoxil in patients with chronic hepatitis B due to lamivudine-resistant HBV (M. Peters, H. W. Hann, P. Martin, E. Heathcote, P. Buggisch, A. E. Moorat, M. Sullivan, K. Kleber, R. Ebrahimi, S. Xiong, and S. Brosgart, Oral Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Dis., abstr. 646, 2002). Patients were randomized to receive either adefovir dipivoxil (10 mg), or adefovir dipivoxil (10 mg) plus lamivudine (100 mg), or lamivudine (100 mg). After 16 weeks, patients in both the adefovir dipivoxil monotherapy and combination adefovir dipivoxil plus lamivudine groups achieved a similar reduction in serum HBV DNA levels from baseline (2.86 and 2.87 log₁₀ copies per ml), compared to no change in patients treated with 100-mg lamivudine monotherapy (Fig. 17). After 32 weeks of treatment, 32% of the patients who had received adefovir dipivoxil monotherapy had lost the lamivudine resistance mutation M204V/I (located within the YMDD motif).

View larger version (12K): [in this window] [in a new window]   FIG. 17. Median changes in serum HBV DNA levels in patients in a 48-week, randomized, double-blind, placebo-controlled multicenter study (study 451). Patients with chronic hepatitis B due to lamivudine-resistant HBV were randomized to receive either 10 mg of adefovir dipivoxil (ADV), 10 mg of adefovir dipivoxil plus 100 mg of lamivudine (ADV + LAM), or 100 mg of lamivudine (LAM) (Peters et al., Oral Presentations 37th Annu. Meet. Eur. Assoc. Study Liver Dis., abstr. 646, 2002).