16th International HIV Drug Resistance Workshop 12-16 June 2007, Barbados

CHARACTERIZATION OF MARAVIROC RESISTANCE IN PATIENTS FAILING TREATMENT WITH CCR5-TROPIC VIRUS IN MOTIVATE 1 AND MOTIVATE 2

Antivir Ther. 2007; 12:S12 (abstract no. 10)

J Mori¹, M Mosley¹, M Lewis¹, P Simpson¹, J Toma², W Huang², J Whitcomb², G Ciaramella¹ and M Westby^1
1Pfizer Global Research and Development, Kent, UK; ²Monogram Biosciences, California, USA

BACKGROUND: Maraviroc-resistant viruses generated in vitro appear able to use either maraviroc-bound CCR5 or free CCR5 as a coreceptor for cell entry. This mechanism of resistance is characterized phenotypically by dose–response curves with a reduced maximal percentage inhibition (MPI). To identify phenotypic and genotypic markers of maraviroc resistance, in vivo, samples from 37 patients in the Phase III clinical studies (MOTIVATE 1 and 2) who failed therapy with a CCR5-tropic virus were analysed.

METHODS: Paired baseline and on-treatment samples were tested for maraviroc susceptibility using an Envelope (Env)-pseudotyped virus assay (PhenoSense Entry^™ assay). Samples were identified for further investigation if one of three pre-defined criteria were met: plateau in MPI <95%, IC₅₀ fold-change compared to reference virus (FC) outside the normal range (>1.95), or >twofold change in IC₅₀ between baseline and on-treatment samples. In these cases, susceptibility testing, gp160 sequencing and tropism confirmation were performed on 12 Env clones from each baseline and on-treatment sample. For five patients, Env genes were cloned into NL4-3, producing replicationcompetent viruses for testing in PBL.

RESULTS: No plateaus in MPI <95% were seen for any of 37 baseline viruses or for on-treatment virus from the 25 placebo patients. In contrast, plateaus in MPI <95% were seen for on-treatment samples from 4/12 patients following failure on blinded maraviroc and 2/3 patients subsequently followed onto open-label maraviroc. There was no significant difference in on-treatment maraviroc IC₅₀ FCs between treatment groups (P=0.13, ANCOVA), although the on-treatment virus from one patient failing maraviroc demonstrated a small (threefold) shift from baseline. Clonal analysis confirmed results for the original population samples and results obtained in PBL were consistent with observations in the PhenoSense assay. Amino acid changes from baseline were identified in the V3 loops of samples yielding a plateau in MPI. Site-directed mutagenesis confirmed the importance of the V3 mutations in conferring the maraviroc phenotype. The amino acid changes differed between patients, reflecting the heterogeneity in gp160 sequence. There was no evidence of cross-resistance with the fusion inhibitor enfuvirtide.

CONCLUSIONS: These results validate the use of a reduced MPI as a phenotypic marker of maraviroc resistance in vivo and identify V3 loop mutations as likely conferring the resistance phenotype.

2007-06-12
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