MK-2048
The discovery of a low-to-moderate genetic barrier of resistance with first generation INIs led to efforts to produce second generation INSTIs with activity against RAL-resistant viruses. Optimization of tricyclic 10-hydroxy-7,8-dihydropyrazinopyrrolopyrazine-1,9-dione compounds led to the development of MK-2048 [
51] (Figure
1), which demonstrates a EC
95 < 50 nM when assayed in 50% human serum and possesses a favorable pharmokinetic profile in dogs and rats [
52]. MK-2048 was subsequently shown in tissue culture and biochemical assays to be effective against RAL- and EVG- resistant viruses [
51‐
55], with only slightly diminished effectiveness against viruses containing at least two of the following mutations: E138K, G140S and Q148R [
51‐
55]. Selection studies in culture with MK-2048 did not select for previously recognized mutations but instead selected a novel substitution at position G118R that, in concert with E138K, conferred approximately eight-fold resistance to MK-2048 [
56]. Despite its favorable resistance profile, MK-2048 has a poor pharmacokinetic profile and its clinical development has been arrested. However, it has potential as a candidate microbicide for prevention of HIV infection [
57]. It continues to be studied as a prototype second generation INI and has also recently shown effectiveness in the treatment of human T-lymphotropic virus type 1 in culture without causing significant toxicity in target cells [
58].
Dolutegravir
Dolutegravir (DTG) (S/GSK 1349572) is currently in phase III clinical trials (for structure, see Figure
1). It was discovered at Shionogi Pharmaceuticals in Japan and is now being developed by a Shiniogi-ViiV Healthcare-GlaxoSmithKline joint venture [
59,
60]. DTG is a promising HIV INI candidate that specifically inhibits the strand transfer reaction with recombinant purified integrase [
60]. Inhibition of the integrase strand transfer reaction by DTG has been confirmed in studies with live virus, which demonstrated an accumulation of 2-long terminal repeat (2-LTR) circles in treated cells at DTG concentrations < 1,000-fold of those that caused cell toxicity [
61,
62]. DTG also demonstrated efficacy against most viral clones resistant to RAL and EVG and against clinical isolates of HIV-1 and HIV-2, although some viruses containing E138K, G140S or R148H mutations possessed diminished susceptibility to DTG [
60,
63‐
65]. Double mutants containing combinations of E138K, G140S and R148H had a FC > 10 for DTG, but this was favorable when compared to RAL, which yielded a FC of > 330 and > 140, respectively.
In vitro combination antiviral studies showed that DTG did not increase toxicity when used in combination, but had a synergistic effect with each of EFV, nevirapine, stavudine, abacavir, LPV, amprenavir and enfuvirtide as well as an additive effect in combination with maraviroc. The hepatitis B virus drug adefovir and the hepatitis C virus drug ribavirin had no effect on the efficacy of DTG [
65], allowing for its potential use in treating co-infections.
The pharmacokinetic profile of DTG allows once-daily dosing without pharmacokinetic boosting. This is based on a long unboosted half-life (13 to 15 hours) with trough levels of DTG being much higher than the
in vitro IC
90 [
66]. The side-effects of DTG in volunteers with HIV infection were similar to those of placebo in phase I clinical trials [
66].
Phase IIa randomized double blind trials provided vital evidence of the anti-HIV effect and potency of DTG [
67,
68]. Notably, 35 ARV-experienced INI-naïve patients, who were not receiving therapy, and whose plasma HIV-1 RNA levels ranged from 3.85 to 5.54 log copies/mL, received once-daily doses of 2 mg, 10 mg or 50 mg DTG or placebo for 10 days. More than 90% of patients who received DTG, irrespective of dose, had a decline in viral load to < 400 copies/mL while 70% of patients in the 50 mg arm achieved undetectable viremia. In contrast, the placebo group showed an average increase in viremia. No serious adverse effects were reported in this trial, with headaches and pharyngolaryngeal pain being the most commonly reported consequence [
67].
In the SPRING-1 double blind dose-ranging phase II trials, 205 ARV-naïve patients with HIV, with CD4
+ cells > 200 cells/mm
3 and HIV-1 RNA > 1,000 copies/mL, were treated once daily with DTG (n = 155) at10 mg, 25 mg or 50 mg doses or 600 mg EFV (n = 50) combined with background therapy of TDF/FTC or abacavir/3TC [
69]. More than 90% of all participants in the DTG arm had undetectable viremia after 24 weeks of treatment, establishing the non-inferiority of DTG to EFV in an NRTI or non-NRTI background and also showing that DTG was at least as safe as EFV.
No primary INI resistance mutations have yet been reported for DTG either in culture or in the clinic. Tissue culture selection studies over 112 weeks identified, in order of appearance, viruses harboring T124S/S153F, T124A/S153Y, L101I/T124A/S153F and S153Y by week 84. Although these mutations persisted throughout serial passaging, they did not confer high-level resistance to DTG [
65]. Position 124 of IN is modestly polymorphic and S153F/Y had previously been described in EVG selection studies [
70]. Despite an apparently high genetic barrier for resistance, selection, recent tissue culture and biochemical studies report that a R263K mutation in IN may confer modest resistance to DTG [
71].
It has been suggested that DTG enjoys a high barrier for resistance due to a tighter binding of DTG to IN compared to RAL and EVG [
72]. Assays also showed that DTG exhibited tighter binding and had a longer dissociative half-life from IN than either RAL or EVG [
73].
In this model, a direct relationship existed between the half-life of binding and the inhibitory potential of INIs when the binding half-life (t
1/2) was below 4 hours. A > 3 FC in regard to drug resistance, relative to the wild-type, was observed when the t
1/2 dropped below 1 hour [
72]. In assays with wild-type enzymes, the t
1/2 of DTG, RAL and EVG were 71, 8.8 and 2.7 hours, respectively. The fact that RAL and EVG have a shorter t
1/2 than DTG suggests that resistance mutations that affect binding of RAL and EVG might also be more likely to compromise antiviral potency. As an example, the Y143CHR mutations have been shown to compromise interactions between IN and RAL but not those between IN and DTG or between IN and EVG [
74]. This is further supported by data on mutations that have been shown to significantly reduce t
1/2, E92Q/N155H, E138K/Q148R and G140S/Q148R, and significantly reduce antiviral potency [
72]. This hypothesis had been previously suggested for MK-2048, which also has a relatively high barrier for resistance, as it also has a slower off-rate (t
1/2 = 32 hours) for IN compared to RAL (t
1/2 ≥ 7.3 hours) [
73].
The use of DTG in INI-salvage therapy is being investigated in an ongoing study called VIKING. The latter is a phase II single arm study investigating the feasibility of replacing RAL with DTG in patients experiencing failure due to RAL-resistant viruses [
59]. Participants (n = 27) were switched from their previous RAL-containing regimens to receive DTG 50 mg once daily for 10 days and were then prescribed other active drugs over a period of 23 weeks. Eighteen of the study participants had INI- resistant viruses belonging to the Y143, Q148 and N155 pathways prior to initiation of the study. After 10 days of DTG monotherapy, all participants harboring viruses in the Y143 and N155 pathways attained a mean HIV-1 RNA decrease of approximately 1.8 log copies/mL compared with approximately 0.7 log copies/mL for viruses harboring G140S/Q148HRK double mutations. None of the viruses harboring Q148HRK plus two or more additional mutations experienced a decrease of ≥ 0.7 log copies/mL, indicating a degree of resistance on the part of Q148HRK viruses to DTG. This trial nonetheless provided proof-of-principle for the use of DTG in RAL-experienced patients infected by subtype-B viruses harboring position Y143 and N155 mutations.
In order to model the effects of DTG in RAL-experienced patients, several serial passaging studies have been carried out and shown that the presence of the N155H and Y143CHR resistance did not lead to development of additional resistance mutations under DTG pressure nor to a substantial decrease in DTG susceptibility [
62,
64]. In contrast, the presence of Q148HRK mutations did lead to further mutations and > 100 FC for DTG susceptibility relative to wild-type in subtype B viruses [
63,
65]. Interestingly, Q148HRK mutations did not affect susceptibility to DTG in HIV-1 subtype C and HIV-2 isolates [
65,
75,
76]. An ongoing trial termed SPRING-2 will evaluate the use of once-daily DTG versus twice-daily RAL in treatment-naïve patients. A Phase III trial termed SAILING will compare once-daily versus twice-daily DTG in ARV-experienced INI-naïve participants with HIV [
77].