Appearance of Other Mutations in NFV-Treated Subjects Who Developed D30N and/or L90M Mutation

• Baseline genotype was available for 30/31 NFV-treated subjects whose rebound sequences displayed the D30N or L90M mutation.

• Coincident with the appearance of D30N or L90M, 16/30 rebound isolates contained at least one additional primary or secondary mutation that was not present at baseline. These included L10F/I/V, M36I, M46I, L63P, A71TV, G73S, V77I, V82A/T, I84V and N88D.

• New secondary mutations appeared in 12 rebound isolates containing the D30N and 4 rebound isolates containing the L90M mutation.

Appearance of Polymorphisms/Secondary Mutations in LPV/r-Treated Subjects

• Baseline genotype was available for 37/40 subjects with rebound genotype. Genotype data for all 40 subjects are provided in Table 5.

• Isolates were examined for the presence of any of the following polymorphisms/secondary mutations:10, 20, 24, 33, 36, 53, 54, 63, 71, 77 and 88.³

• Each of the rebound sequences for 30/37 (81%) subjects displayed no new secondary mutations.

• The remaining 7/37 rebound sequences demonstrated one secondary mutation that was not present at baseline, including L10F (1), M36I (4), L63P (1), and A71T (1).

• At rebound, 2 subjects no longer had a secondary mutation that had been present at baseline.

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Responders Had Better Adherence Than Non-responders

• For this analysis, a total of 171 subjects (65 LPV/r-treated subjects and 106 NFV-treated subjects) were classified as non-responders (VL >400 copies/mL at least once at Week 24, 32, 40, 48 or 60), and 419 subjects were classified as responders (VL <400 copies/mL on all values obtained between Week 24 and 60).

• Responders demonstrated statistically significantly better adherence than non-responders (Table 6).

• Adherence and exposure to viral replication were compared between the 40 LPV-treated subjects and 84 NFV-treated subjects for whom genotype data were available.

• Adherence was statistically significantly higher in responders than in non-responders (Table 6).

• Adherence was similar between the two treatment groups (Table 6).

• Exposure to viral replication (baseline viral load, viral load at the time of genotype, days with VL >400 and viral load area under the curve) was comparable between the two treatment groups.

DISCUSSION

A semi-quantitative pharmacological model may account for the lack of resistance to lopinavir observed in this study (Figure 2). During periods of adherence, plasma levels of LPV remain well in excess of the serum-adjusted IC₅₀ against wild-type HIV (high inhibitory quotient), and viral replication of both the wild-type virus and any pre-existing viral mutants are likely to be suppressed. Drug concentrations that lie between the IC₅₀ values for the wild-type and mutant viruses are expected to provide the greatest selective replication advantage for the mutant (zone of highest selective pressure). During periods of non-adherence, plasma drug levels decline through the zone of highest selective pressure. As drug levels continue to decline below this concentration zone, overall replication increases, and, in the absence of drug, the wild-type virus has a fitness advantage over any mutants. Understanding the selection of resistance in vivo requires an estimation of the time during which significant selective pressure exists (i.e., how frequently and how rapidly plasma drug concentrations decline through the zone of highest selective pressure).

Single mutants display <3-fold reduced susceptibility to lopinavir. ^4,5 Based on steady-state pharmacokinetic determinations, the median estimated time for plasma levels of LPV to decay to the upper boundary of zone of highest selective pressure is 20.5 hours. This time would approximate missing a scheduled LPV/r BID dose by 8 hours. By this time, plasma concentrations of ritonavir have declined to levels that no longer adequately inhibit the metabolism of lopinavir. Thus, as concentrations of lopinavir enter the zone of highest selective pressure, the clearance of lopinavir has increased substantially (short half-life). Thus, the estimated median time for LPV concentrations to decay through the zone of highest selective pressure is approximately 3.5 hours (range 2.5-5 hours). Since viral maturation through proteolytic processing by active (uninhibited) wild-type HIV protease is not instantaneous (estimated half-life up to 1.5 hours for production of mature, infectious particles)⁶ and the kinetics of HIV protease containing primary mutations can be substantially impaired compared to wild-type protease,⁷ the selective production of infectious virus containing primary resistance mutations may be minimal during this period.

CONCLUSIONS

• Protease inhibitor resistance was not observed in any LPV/r-treated subject through 60 weeks of observation. In contrast, isolates from 37% of NFV-treated subjects with detectable VL demonstrated protease inhibitor resistance.

• Resistance to 3TC was observed significantly more frequently in NFV-treated vs. LPV/r-treated subjects (82% vs. 38%, p<0.001).

• The lack of protease inhibitor resistance observed in this study is consistent with a pharmacologic barrier to resistance produced by the high inhibitory quotient and unique terminal pharmacokinetics of LPV/r. The substantial difference in the incidence of resistance between LPV/r and NFV may have important implications for the development of ARV treatment strategies.

REFERENCES

1. Neu, H. The inhibitory quotient. A method for interpreting minimum inhibitory concentration data. JAMA, 1981; Oct. 2, 246 (14): 1575-8.

2. Bertz, R et al. Multiple Dose Pharmacokinetics (PK) of ABT-378/ritonavir (ABT-378/r) in HIV+ Subjects. 39th Interscience Conference on Antimicrobial Agents and Chemotherapy. San Francisco, USA, 1999 (Abstract 0327).

3. Hirsch, M.S. et al. Antiretroviral drug resistance testing in adults with HIV infection: Implications for clinical management. International AIDS Society-USA Panel. JAMA 1998;279: 1984-91.

4. Molla A, Vasavanonda S, Kumar G, et al. Human serum attenuates the activity of protease inhibitors toward wild-type and mutant human immunodeficiency virus. Virology 1998; 250: 255-62.

5. Carrillo A, Stewart K, Sham HL, et al. In vitro selection and characterization of human immunodeficiency virus type 1 variants with increased resistance to ABT-378, a novel protease inhibitor. J. Virology 1998; 72:7532-7541.

6. Kaplan, A, Manchester M, and Swanstrom R. The activity of the protease of human immunodeficiency virus type 1 is initiated at the membrane of infected cells before the release of viral proteins and is required for release to occur with maximum efficiency. J. Virology 1994; 68:6782-6786.