December 5, 2012
Table of Contents
What new HIV medications do we have to look forward to over the next few years? How will these newer drugs improve upon the older ones? To shed some light on these questions, Roy Gulick, M.D., provided an overview at ID Week 2012 of drugs in development.
Since the first HIV medication, zidovudine (AZT, Retrovir), was approved in 1987, 26 other antiretrovirals have been made available in the U.S. for treating HIV -- a history that Gulick recapped in song during this conference. Our best regimens today are potent, convenient and relatively non-toxic.
However, according to Gulick, there is potential to make medications even better than those we have today. Newer drugs should build upon some of these aspects, he said:
The list of drugs in the pipeline continues to be full of antiretroviral agents, whether they are in early development or undergoing clinical trials. Gulick highlighted six of the most promising drugs.
GS-7340 is an investigational nucleoside agent that is a prodrug of the approved formulation of tenofovir (TFV, Viread). A prodrug is a medication that, when metabolized in the blood, breaks down into the active form of the compound. The NRTI sold under the brand name Viread is actually tenofovir disoproxil fumarate (TDF), a prodrug that breaks down into tenofovir.
GS-7340's antiretroviral activity was first presented in a study by Martin Markowitz, M.D., and others at CROI 2011. In a small, 14-day study, Markowitz and his team found that GS-7340 performed slightly better than the "old" TDF, with greater decreases in HIV RNA at lower dosages (GS-7340 at 50 or 150 mg vs. TDF at 300 mg).
These findings were further supported by study results from Peter Ruane, M.D., and others at CROI 2012. Ruane and his team compared GS-7340 (at dosages of 8, 25 and 40 mg) with TDF (at 300 mg) in 38 treatment-naive patients over 10 days of monotherapy. GS-7340 again performed better than TDF, showing 0.76, 0.94 and 1.08 log reductions in HIV RNA, respectively, while TDF only showed a 0.48 log reduction in HIV RNA. The findings were statistically significant for the 25-mg (P = .017) and 40-mg (P = .01) dosages.
Ruane and his group also found that the plasma concentrations of tenofovir, when the prodrugs were metabolized, were 10 to 100 times higher for TDF than for any of the three dosages of GS-7340. This finding suggests that, because GS-7340 delivers less compound to target tissues, it could reduce toxicity levels in the organs, Gulick said.
On the other hand, when comparing intracellular concentrations of tenofovir in peripheral mononuclear cells like lymphocytes (which is where we want the drugs to be), GS-7340 achieved up to 20 times higher levels than TDF, Gulick noted.
In both of these studies, GS-7340 was generally well tolerated and no serious adverse events were reported.
A third study of GS-7340 was presented at ICAAC 2012. It found that GS-7340 had high potency against 26 HIV-1 isolates representing 7 subtypes. The drug also showed high potency against three HIV-2 isolates. In addition, GS-7340 maintained its viral potency longer than TDF, showing its better stability.
Further GS-7340 studies are in progress. Particularly because of its low dosage and high potency, it can be readily co-formulated with other agents. Gulick pointed out two studies exploring such coforumulations. The first study will compound GS-7340 with emtricitabine (FTC, Emtriva) plus elvitegravir (EVG) plus cobicistat, and compare that to elvitegravir/cobicistat/emtricitabine/tenofovir (Stribild). The second study will compound GS-7340 with emtricitabine, darunavir (Prezista) and cobistat (which would be the first one-pill, once-a-day protease inhibitor-based regimen), and compare that to tenofovir/emtricitabine (Truvada) plus darunavir plus cobicistat.
Of the six highlighted compounds, Gulick stated that dolutegravir is the furthest along in development. Dolutegravir is an investigational integrase inhibitor, but it has distinguished itself from the two approved integrase inhibitors, raltegravir (Isentress) and elvitegravir. Dolutegravir has a long half-life of 15 hours, indicating it can be taken once a day. Gulick emphasized that it does not require pharmacokinetic boosting. He noted that resistance does occur, but that dolutegravir showed activity against raltegravir- and elvitegraivr-resistant viral strains.
Its antiviral potency was shown in a phase-2a study by Sherene Min, M.D., and others. In 28 treatment-naive patients receiving either 2, 10 or 50 mg of dolutegravir, there was an average of a 1.51 to 2.46 log reduction in viral load after only 10 days of once-daily dosing. Seven of the 10 patients receiving 50 mg dosages achieved a viral load less than 50 copies/mL. Min and her team reported low pharmacokinetic variability and good short-term tolerability (the most common side effects were diarrhea, fatigue, and headache; adverse events were mild to moderate in severity).
According to Gulick, phase-3 results are complete and will be submitted to the U.S. Food and Drug Administration by the end of the year.
One of the phase-3 studies, known as SPRING 2, found dolutegravir to be non-inferior to raltegravir. The study followed 827 treatment-naive patients with a viral load above 1,000 copies/mL over 48 weeks. They were given either 50 mg of dolutegravir or 400 mg of raltegravir. Both groups were successful at achieving viral loads below 50 copies/mL (88% for dolutegravir and 85% for raltegravir). Both drugs were also very well tolerated, with only 2% in each group having to discontinue treatment because of adverse events.
Furthermore, a dolutegravir-based regimen consisting of abacavir/lamivudine (Epzicom, Kivexa) plus dolutegravir was actually found to be superior to tenofovir/emtricitabine plus efavirenz (Sustiva, Stocrin), according to the results of a companion phase-3 study by Sharon Walmsley, M.D., and others. In 822 treatment-naive patients studied over 48 weeks, 88% of the dolutegravir group achieved a viral load below 50 copies/mL, compared to 81% of the efavirenz group. Gulick pointed out that the difference was because of tolerability: 10% of the efavirenz group discontinued treatment because of adverse events, compared to just 2% of the dolutegravir group. He commented that this would mark the first real challenger to efavirenz's long-held dominance in treatment-naive studies with a 48-week primary endpoint.
In terms of renal safety, the study found dolutegravir did interfere with tubular secretion of creatinine. However, Gulick noted, the increase in creatinine was only about .1 to .15 mg/dL, and occurred only within the first two weeks after starting dolutegravir, then stabilized over the rest of the 48-week study. As the Walmsley study noted, dolutegravir does not affect actual glomerular filtration rate.
In terms of resistance, dolutegravir appears to have a higher barrier to resistance than the other integrase inhibitors. In the Walmsley study, among both the dolutegravir and efavirenz groups, only 4% experienced virologic failure (18 and 17 individuals, respectively). Of the nine in each group that had genotypic test results available, "You see no nucleoside and no integrase mutations in the dolutegravir group. And as you would expect in the efavirenz [regimen], there were some nucleoside and non-nucleoside mutations detected," Gulick stated.
Because dolutegravir showed activity against elvitegraivr- and raltegravir-resistant viral strains, as shown in a study by Masanori Kobayashi and others, Joseph Eron, M.D., and others studied the use of dolutegravir for patients who had developed resistance to raltegravir. Their pilot study, known as VIKING, followed 51 patients with three or more class resistances, including demonstration of raltegravir mutations. The patients were given 50 mg of dolutegravir, either once or twice a day, for 10 days. A virologic response was defined as either a viral load below 400 copies/mL or a 0.7 log reduction. As Gulick explained, "The best responses were in the twice-a-day group. Whether you looked at all patients, those with the specific Q148 or other mutations, you can see response rates, over a short 10 days of therapy, exceeding 90%."
The VIKING study went on to follow the patients over 24 weeks. After the initial two weeks, the patients added an optimized background regimen. These follow-up results were presented by Vincent Soriano, M.D., Ph.D., at the 2011 European AIDS Conference. Soriano and his team found that by the end of the 24-week period, 41% of the once-a-day group and 75% of the twice-a-day group were able to re-suppress their viral load below 50 copies/mL.
Further studies of dolutegravir in the setting of other integrase inhibitor resistance are ongoing.
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