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SPI-256: a novel highly potent HIV protease inhibitor
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"....SPI-256 retained nanomolar potency against all selected mutants (range 9-170 nM). The IC50 of SPI-256 against the virus containing I50V & L10F, and the gag mutation, A431V increased about 5 fold compared to wild-type.....the selection of virus resistant to SPI-256 was a very slow process and required multiple passages with small incremental increases of concentration. These results support the potential use of SPI-256 in both first-line and salvage therapy regimens."
Reported by Jules Levin
15th Intl HIV Drug resistance Workshop, June 13-17, 2006, Sitges, Spain
E Afonina, S Gulnik, H Yokoe and J Erikson
Sequoia Pharmaceuticals, Inc, Gaithersburg, MD, USA
The first significant presentation on this new PI was a poster at CROI 2006.
BACKGROUND: SPI-256 is highly active against WT-HIV with IC50 values
METHODS: In vitro selection experiments were performed by passaging WT-HXB2 HIV or MDR-HIV patient isolates (M4 & M7) in MT4 cells, in the presence of increasing concentrations of SPI-256. Both M4 & M7 contained numerous PI-resistance mutations, were sensitive to SPI-256 (IC50s of 19 & 1 nM, respectively), and were highly cross-resistant to FDA approved PIs (IC50 range 0.08-8 uM). Viral RNA was extracted from passaged virus and subjected to populational & clonal sequencing. Phenotypic characterization of selected mutant strains was done using a cell-based MTT assay.
RESULTS
Selection experiments were initiated using a drug concentration that was at the phenotypic IC50 value, and drug concentrations were increased in small increments often not more than 30%. About 136 days and 18 passages were necessary to establish a virus population growing at 90 nM of SPI-256 (45 x IC50).
Genotypic analysis of selected virus revealed the presence of L10F, M46I, and I50V mutations in the protease gene with additional mutations in p7/p1 and pi/p6 cleavage sites. Selection of resistant virus starting from M4 & M7 viruses also required multiple passages: a 40-fold increase from the starting selection concentration was achieved after 50 days. Escape viruses from both the M4 & M7 selections contained I50V. Further passaging of WT virus led to the addition of I47V protease substitution. Mutant viruses selected using WT, M4 and M7 viruses were used for phenotypic susceptibility testing. SPI-256 retained nanomolar potency against all selected mutants (range 9-170 nM). The IC50 of SPI-256 against the virus containing I50V & L10F, and the gag mutation, A431V increased about 5 fold compared to wild-type.
CONCLUSION: I50V appeared to be a common mutational escape pathway to SPI-256 for WT as well as highly divergent, MDR PI-resistant viruses. However, the selection of virus resistant to SPI-256 was a very slow process and required multiple passages with small incremental increases of concentration. These results support the potential use of SPI-256 in both first-line and salvage therapy regimens.
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