Mechanism of Action
Ibalizumab-uiyk, a recombinant humanized monoclonal antibody, blocks HIV-1 from infecting CD4+ T cells by binding to domain 2 of CD4 and interfering with post-attachment steps required for the entry of HIV-1 virus particles into host cells and preventing the viral transmission that occurs via cell-cell fusion.
Ibalizumab-uiyk Does Not Impact CD4 Function
The binding specificity of ibalizumab-uiyk to domain 2 of CD4 allows ibalizumab-uiyk to block viral entry into host cells without causing immunosuppression. Epitope mapping studies indicate that ibalizumab-uiyk binds to a conformational epitope located primarily in domain 2 of the extracellular portion of the CD4 receptor. This epitope is positioned on the surface of CD4 opposite to the site in domain 1 that is required for CD4 binding of the MHC class II molecules and therefore does not interfere with CD4-mediated immune functions. Additionally, ibalizumab-uiyk does not interfere with gp120 attachment to CD4.
Antiviral Activity
Ibalizumab-uiyk inhibits the replication of CCR5- and CXCR4-tropic laboratory strains and primary isolates of HIV-1 in phytohemagglutinin stimulated peripheral blood lymphocytes. The median EC50 value (50% effective concentration) for ibalizumab-uiyk against HIV-1 group M isolates (subtypes A, B, C, D, E, or O) was 8 ng/mL (n = 15, range of 0.4 to 600 ng/mL) in cell culture, with lower susceptibility observed in macrophage-tropic HIV-1 strains (BaL, JR-CSF, YU2, and ADA-M). In a single-cycle infection assay, ibalizumab-uiyk inhibited 17 clinical isolates of subtype B with a median EC50 value of 12 ng/mL (range of 8.8 to 16.9 ng/mL; mean 12 ± 3 ng/mL) and a median maximum percentage inhibition (MPI) of 97% (range of 89 to 99%; mean 97 ± 3%). Three CCR5-tropic clinical isolates from subtypes B, C, and D, were inhibited with EC50 values ranging from 59-66 ng/mL and 3 CXCR4-tropic clinical isolates from subtypes B, C, and D, with EC50 values ranging from 44-59 ng/mL.
Antiviral Activity in Combination with Other Antiviral Agents
No antagonism was observed when PBMCs or MAGI-CCR5 cells infected with the subtype B Ba-L or ADA variants of HIV-1 were incubated with ibalizumab-uiyk in combination with the CCR5 co-receptor antagonist maraviroc or when PBMCs infected with the subtype B HT/92/599 variant of HIV-1 were incubated with ibalizumab-uiyk in combination with the gp41 fusion inhibitor enfuvirtide; a nonnucleoside reverse transcriptase inhibitor (efavirenz); nucleoside analog reverse transcriptase inhibitors (abacavir, didanosine, emtricitabine, tenofovir, or zidovudine); or a protease inhibitor (atazanavir).
Antiviral Activity in Antiretroviral-Resistant Virus
Subjects enrolled in TMB-301 were heavily treatment-experienced subjects infected with multidrug resistant HIV-1. Ibalizumab-uiyk inhibited 38 baseline isolates at a median EC50 value of 31 ng/mL (range of 13 to 212 ng/mL; mean 39 ± 35 ng/mL) with a median MPI of 97% (range of 41-100%; mean 91 ± 14%). For 10 subjects in TMB-301 who failed treatment, at the time of failure the median ibalizumab-uiyk EC50 value was 566 ng/mL (range of 148 to >54,900 ng/mL; mean 11,768 ± 21,650 ng/mL) representing an EC50 value shift of >18-fold. For the HIV-1 derived from the same subjects, the median MPI was 55% (range of 43-72%; mean 56 ± 8%) representing a 42 percentage point reduction.
Decreased Susceptibility
Decreased susceptibility to ibalizumab-uiyk, as defined by a decrease in MPI, has been observed in some subjects experiencing virologic failure and may be associated with genotypic changes in the HIV-1 envelope coding sequence that results in the loss of potential N-linked glycosylation sites (PNGS) in the V5 loop of gp120. The clinical significance of decreased susceptibility to ibalizumab-uiyk has not been established.
Cross-Resistance
Phenotypic and genotypic test results revealed no evidence of cross-resistance between ibalizumab-uiyk and any of the approved classes of anti-retroviral drugs (CCR5 co-receptor antagonists, gp41 fusion inhibitors, integrase strand transfer inhibitors [INSTIs], non-nucleos(t)ide reverse transcriptase inhibitors [NNRTIs], nucleos(t)ide reverse transcriptase inhibitors [NRTIs], or protease inhibitors [PIs]). Ibalizumab-uiyk is active against HIV-1 resistant to all approved antiretroviral agents and exhibits antiretroviral activity against R5-tropic, X4-tropic, and dual-tropic HIV-1.
Decreased susceptibility to ibalizumab-uiyk following multiple dose administrations of ibalizumab-uiyk has been observed in some subjects. Cell culture studies performed with HIV-1 variants with reduced susceptibility to ibalizumab-uiyk indicate that phenotypic changes associated with resistance to ibalizumab-uiyk do not alter susceptibility to other approved agents and do not result in the selection of CD4-independent viral isolates.
CD4 Polymorphisms and Ibalizumab-uiyk Activity
CD4 polymorphisms reported in public databases were analyzed to determine if any naturally occurring amino acid substitutions in the CD4 molecule from different human populations would potentially impact the antiviral activity of ibalizumab-uiyk. None of the known CD4 polymorphisms are likely to have an impact on ibalizumab-uiyk binding to CD4.