The 2003 HIV Drug Resistance Workshop included detailed information on resistance profiles of new protease inhibitors, NNRTIs and nucleoside analogues, as well as information about new potential classses of antiretrovirals.
Elvucitabine
Achillion Pharmaceuticals presented data on elvucitabine (ELV), an L-nucleoside analogue active against HIV and HBV. Dosed once a day with a half-life of over 20 hours, ELV has demonstrated in vitro activity against viral strains resistant to all current classes of HIV drugs including NRTIs, NNRTIs and PIs.
Two studies presented an in vitro profile of ELV-induced resistance and the impact of ELV with two dosing regimens in patients harbouring M184V mutations. The latter study involved 56 patients receiving HAART therapy with a baseline genotype that included the M184V mutation. 46 completed the 28 days of observation therapy with four patients discontinuing because of rash or neutropenia. Patients were treatment-experienced and all had mutational profiles that included one primary nucleoside resistance mutation (in addition to M184V) and primary resistance to NNRTIs or PIs. Their viral load (VL) ranged from 1,000 to 30,000 copies (mean VL was 10,300 copies/ml). The mean CD4 count was 471 cells/mm3.
Patients were randomised to receive either 50mg ELV a day (25 patients), 100mg ELV a day (25 patients) or continued 3TC 300mg qd (10 patients). After 4 weeks, patients had the option of continuing ELV if they wished. The results showed antiviral activity in both ELV groups with mean declines at 28 days of -0.67 and –0.78 log10 copies/ml in the 50 and 100mg groups respectively, compared to an increase of +0.01 log10 copies/ml in the 3TC group. No correlation was found between baseline genotype and antiviral activity.
Side effects which occurred after three to four weeks of therapy included: mild headache, GI-related disturbances, mild to moderate maculopapular rash and bone marrow suppression. Effects on hematologic parameters were found to be reversible upon discontinuation of treatment. Myelosuppression was largely related to the higher dose of ELV. Triphosphate (TP) levels were not discussed but would have helped to explain whether accumulation of TP over time was responsible for the adverse events observed. The researchers noted that ELV could address the need for future NRTIs that do not present the limiting mitochondrial toxicities currently associated with d-NRTIs such as d4T and ddI (Dunkle)
A concern was raised at the meeting regarding observed reductions in CD4 cell count and a potential cytopathic effect of the compound.
A second ELV study was specifically designed to evaluate resistance mutations selected by ELV in vitro. The study demonstrated two key mutations at M184I and D237E resulting in reduced susceptibility and cross resistance to 3TC but not to other NRTIs. No switch to the M184V mutation was observed in vitro from an initial mutation at M184I. The researchers posit not only a unique resistance profile with ELV but one that may confer hypersensitivity to AZT and 3TC, although this remains to be confirmed clinically (Rice).
TMC114
TMC114-C207 is a 14 day Phase IIa study designed to evaluate antiviral activity, safety and tolerability of TMC114, the new protease inhibitor from Tibotec-Virco. A number of presentations profiled TMC114 and its phenotypic and genotypic resistance characteristics, and explained why resistance to TMC114 may be difficult to define.
In the first study, 50 patients experiencing NRTI failure and PI regimens enrolled on one of three doses: TMC114 300mg administered with riotnavir 100mg bid, 600mg/100mg bid or 900/100 qd as a substitution for their current PI. A control group retained their existing regimens. 46% of patients were resistant to all currently approved PIs with only 27% sensitive to two or more PIs. All primary PI mutations were observed at baseline in at least one sample (except for mutations I50L and V82S). All patients switched to an investigator-selected HAART regimen after 14 study days. Antiviral activity was found to be -1.35log10 copies/ml compared to +0.02log10 copies/ml in the control group. No significant differences were observed between the three dosing arms and no mutational patterns influencing response to treatment with TMC114 could be detected in this study (De Meyer).
A further study used crystal structures of TMC 114 and other protease inhibitors in complex with HIV protease to demonstrate the binding of current PIs compared to the tighter binding of TMC114. Very elegant graphics illustrated how currently licensed PIs spill over the binding pocket, whereas TMC144 fits inside the pocket without engaging other areas that might be able to adapt through conformational changes – a factor hypothesised to encourage the emergence of resistant variants (King).
Tipranavir
Tipranavir (TPV), the Boehringer-Ingelheim non-peptidic PI (NPPI) has been in gestation for quite some time. The latest data presented at the workshop focused on establishing an inhibitory quotient (IQ) for TPV, characterising mutational profiles from Phase II studies and in vitro susceptibility to inhibition by mixtures of PI that include amprenavir (APV) and lopinavir (LPV).
Ninety one patients from trials BI 1182.4 and BI 1182.2 who were experiencing single or multiple PI failure received various doses of TPV and ritonavir. Fifty patients with single PI experience were given two new NRTIs plus either high or low dose TPV/ritonavir bid. Forty one multiple PI-experienced and NNRTI- naïve patients received EFV, one new NRTI and either high or low dose TPV/r. On treatment genotypes were available for 63 patients. At study entry, mean PI mutations were 10 in the first study and 12 in the second BI study. They found that whilst one or two universal PI associated mutations (UPAMs: defined as mutations at codons 33, 82, 84 or 90) decreased susceptibility to currently available PIs, >2 UPAMS may be required for reduced susceptibility to TPV. Three of the five patients in study 1182.2 who experienced treatment failure had two UPAMs at baseline and later acquired a third whilst another had three UPAMs at baseline. The researchers conclude that this low level of resistance may be explained by reduced viral fitness conferred by treatment with TPV (Hall).
TMC 125
Tibotec and Virco reported on the activity of the new NNRTI TMC 125 against a panel of 2630 clinical isolates resistant to at least one NNRTI. 79% were resistant to efavirenz and 69% to all current NNRTIs. At a drug concentration of 10nM, TMC 125 inhibited replication of 76% of samples resistant to all NNRTIs, and 63% of samples with four or more NNRTI-associated mutations had a TMC 125 EC50 below 10nM, indicating that the majority remained susceptible to TMC 125. In the phase I/II study of TMC125 reported last year at the Retroviruses Conference, Tibotec researchers found that TMC125 monotherapy in patients who had already experienced failure of one NNRTI did not select for further NNRTI resistance. No loss of TMC 125 susceptibility was seen during the 14 day study (de Bethune; Vingerhoets).
Ribonuclease H – an alternative target to inhibit reverse transcription
Ribonuclease H (RNase H) is essential for retroviral replication, but as yet no RNase H inhibitors are in pre-clinical development. Researchers at Pittsburgh University have developed a fluorescence-based assay for HIV RT RNase H that enables high-throughput screening for inhibitors of this potential target. They found that certain analogues of mappicine act as potent inhibitors of HIV-1 RT-associated RNase H.
A library of 560 mappicine analogues prepared by fluorous-tagged combinatorial synthesis and screened, led to the identification of 55 additional inhibitors. Of these, mappicine 756 showed very good antiviral activity against WT HIV-1 IIIB strain (EC50 2.5uM) and virtually no cytoxicity as assessed in several cell lines. Mappicine 756 retained activity against virus with high-level resistance to nevirapine (NVP), delavirdine (DLV) and efavirenz (EFV). Resistance to mappicine 756 was localised only in the segment of the RT gene corresponding to the RNase H subdomain at positions 0509R, S513R and L525F and should not confer resistance to existing NRTI mutational profiles. Structural variations of mappicine 756 may now be constructed promoting the development of a new class of antiretroviral compounds (Parniak).
Inhibition of integrase strand transfer
Historical studies have shown that inhibitors of the integrase strand transfer reaction can inhibit integration and HIV-1 replication by removing active site metals in integrase.
S-1360 and L870810 are the first compounds in this class to enter pre-clinical studies, demonstrating in vitro inhibition of viral replication with IC95s of 12,000 and 100 nM respectively. Although structurally separate, these compounds have the same mechanisms of action and compete for binding at the same site.
This particular study was designed to assess the potential for cross resistance within this new class. A variety of diketones and naphthyridines related to these compounds were used to select resistance mutations in vitro. All of the inhibitors selected for similar but not identical mutations in integrase and demonstrated marked differences in susceptibility. Some compounds including S-1360 were affected by a wide range of mutations whilst L-870810 displayed an overlapping but more restricted resistance profile. The researchers also identified during the course of the study, other compounds that exhibit discrete resistance profiles and activity against all of the resistant variants identified to date (Hazuda).
References
De Bethune M-P et al. Antiviral activity of TMC125, a potent next-generation non-nucleoside reverse transcriptase inhibitor (NNRTI), against >5000 recombinant clinical isolates exhibiting a wide range of NNRTI resistance. Antiviral Therapy 8: S11, 2003.
De Meyer S et al. TMC-114, a potent next generation protease inhibitor: characterization of antiviral activity in multiple protease inhibitor-experienced patients participating in a phase IIa study. Antiviral Therapy 8: S18, 2003.
Dunkle LM et al. ) Elvucitabine: potent antiviral activity demonstrated in multidrug-resistant HIV infection. Antiviral Therapy 8: S5, 2003.
Hall D et al. Characterisation of treatment-emergent resistance mutations in two Phase II studies of tipravanir. Antiviral Therapy 8: S16, 2003.
Hazuda D et la. The identification of active site mutations that confer resistance to structurally diverse inhibitors of HIV-1 integrase strand transfer supports a general mechanism of phosphotransferase inhibition. Antiviral Therapy 8: S13, 2003.
King N et al. TMC114 binds within the substrate envelope of HIV-1 protease, which could account for its efficacy against multi-protease inhibitor-resistant virus. Antiviral Therapy 8: S19, 2003.
McAllister S et al. Characterization of the impact of genotype, phenotype and inhibitory quotient on antiviral activity of ipranavir in highly treatment-experienced patients. Antiviral Therapy 8: S15, 2003.
Parniak MA et al. Mappicine inhibitors of HIV-1 reverse transcriptase-associated ribonuclease H. Antiviral Therapy 8: S14, 2003.
Rice WG et al. In vitro induction of HIV variants with reduced susceptibility to elvucitabine. Antiviral Therapy 8: S8, 2003.
Vingerhoets J et al. Characterization of resistance before and after short-term therapy with TMC125 in patients with documented non-nucleoside reverse transcriptase inhibitor resistance. Antiviral Therapy 8: S12, 2003.