In both studies, subjects were followed for 56C58 days post-treatment to assess tolerability, pharmacokinetics (PK) and antiviral effects. Unlike the development programs for small-molecule CCR5 antagonists [71C73], the phase 1 programs for ITI214 the CCR5 mAbs did not examine drug-drug or food interactions. infected individuals for 2C3 weeks without appreciable toxicity. Summary CCR5 mAbs have demonstrated broad and potent antiviral activity identified CCR5 mAbs in one of eight hybridoma fusions , while other groups reported screening between 10,000 and 25,000 hybridoma supernatants to identify six to seven novel CCR5 mAbs [25;32]. Epitope specificity The most potent HIV-inhibitory mAbs described to date recognize conformational epitopes. For such mAbs, specificity has been mapped using CCR5 point mutants [25;27;33;34], CCR5 deletants, and/or CCR5 chimeras that contain extracellular regions from homologous chemokine receptors [26;27;33;34]. These approaches have yielded results that are broadly consistent, with CCR5 point mutants providing the greatest precision. For example, independent groups have mapped the epitope for mAb 2D7 to ECL2 using CCR5/CCR2b chimeras [24;27;33;34]. The 2D7 epitope was mapped to ECL2 residues K171 and E172 using CCR5 alanine point mutants [25;27;33;34]. Table 1 lists the epitopes recognized by mAbs that have been mapped using CCR5 point mutants, and the amino acids involved in mAb binding are illustrated in Figure 1. For these mAbs, the dominant epitopes lie within the Nt and ECL2, which are the largest extracellular regions and show significant divergence from mouse CCR5. As illustrated in Figure 1, ECL2 can ITI214 be divided into amino-terminal and carboxy-terminal regions based on patterns of mAb reactivity [27;34]. Table 1 CDKN2A Amino acids implicated in mAb binding to CCR5 as determined using CCR5 point mutantsMutation of the indicated amino acids was reported to reduce mAb binding to CCR5 as determined by flow cytometry. studies examined the antiviral activity of CCR5 mAbs in combination with small-molecule CCR5 antagonists [56C58]. The antibodies examined were PA14, PRO 140 (humanized PA14), 2D7, RoAb13, RoAb14, 2D7 and 45523. The small-molecule CCR5 antagonists included maraviroc, vicriviroc, aplaviroc, SCH-C and TAK-779. Antiviral synergy was reported by each group for most studied combinations of CCR5 mAbs and small-molecule antagonists, and the synergy was attributed to co-binding of CCR5 [56;57]. One notable exception was mAb 45523 used in combination with either maraviroc or aplaviroc, where synergy was not observed due to competition for CCR5 binding . In parallel studies, additive rather than synergistic effects were observed for combinations of small-molecule CCR5 inhibitors [56;57]. The findings provide a rationale to combine CCR5 mAbs and small-molecule antagonists in the clinic and further underscore the mechanistic differences between these classes of CCR5 inhibitors. Synergy also was reported for combinations of CCR5 mAbs that bind distinct epitopes, with the highest synergy observed between Nt and ECL2 mAbs [25;57]. Additive to synergistic effects were reported between CCR5 mAbs and enfuvirtide, a peptide inhibitor of gp41 membrane fusion [56;57]. Additivity was observed between CCR5 mAbs that bind similar or overlapping epitopes. Cross-resistance between CCR5 mAbs and small-molecule CCR5 antagonists ITI214 Viruses resistant to small-molecule CCR5 antagonists were generated by serial passage of virus in the presence of increasing concentrations of inhibitor in vitro. These viruses typically retained an R5 phenotype ITI214 and ITI214 acquired the ability to utilize inhibitor-bound receptor [59C64]. resistance has reflected the emergence of resistant R5 viruses as well as the outgrowth of pre-existing R5X4 viruses [65;66]. In single-cycle antiviral assays, viral resistance to small-molecule antagonists was manifest as a reduction in the maximum percent inhibition at high inhibitor concentrations rather than a change in IC50 [59C63;66], consistent with the view that small-molecule CCR5 antagonists act as allosteric inhibitors [67C69]. Several small-molecule resistant viruses were tested for susceptibility to CCR5 mAbs. Despite demonstrating high-level resistance to the small-molecule CCR5 antagonists, the viruses remained susceptible or even hyper-susceptible to inhibition by CCR5.