Combined doravirine and islatravir cooperate to inhibit NRTI and NNRTI resistant HIV-1 in vitro

Doravirine and islatravir have shown promising activity against multidrug resistant HIV-1. In this study we aimed to evaluate the in vitro susceptibility to doravirine and islatravir as well as their combinatorial activity in a panel of 38 recombinant viruses harboring multiple NRTI and NNRTI mutations. One additional recombinant virus had the M184V mutation alone. According to the IC₅₀ fold-change (FC) values calculated with respect to the NL4-3 wild-type strain, full susceptibility to doravirine was detected in 15/39 (38.5 %) samples, while high-level resistance was mainly associated with specific doravirine resistance mutations. Decreased susceptibility to islatravir was associated with the presence of the M184V/I mutation and increasing numbers of TAMs and NRTI resistance mutations. According to ZIP model of the SynergyFinder Plus tool, the combination of doravirine and islatravir showed additive activity in 37/40 (92.5 %) viruses (including the NL4-3 strain), while synergy and antagonism in one and two cases, respectively. The combination sensitivity score calculated by SynergyFinder Plus indicated a cooperative effect between doravirine and islatravir higher than that observed for the reference NL4-3 strain in 22 (56 %) recombinant viruses. The Multi-dimensional Synergy of Combinations (MuSyC) tool predicted synergy and antagonism in 25 (62.5 %, including NL4-3 virus) and 15 (37.5 %) cases, respectively. MuSyC scores showed a negative correlation with doravirine FC values, number of NRTI mutations and presence of M184V/I, but not with islatravir FC values. Doravirine and islatravir may cooperatively inhibit NRTI and NNRTI resistant viruses despite complex mutational profiles, however the accumulation of resistance mutations may reduce the combinatorial activity.