Table of Contents

Introduction

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:

Improve convenience (reduce dosage frequency, less than once a day).

Improve tolerability and reduce toxicity (even the best drugs today still have some of these issues).

Penetrate reservoirs more effectively (such as the genital tract and central nervous system).

Exploit new targets, thereby improving activity (particularly against drug-resistant viruses).

Improve formulation.

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 (Also Known as Tenofovir Alafenamide, or TAF)

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.

Dolutegravir (DTG)

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.

S/GSK-1265744 (or simply "744")

S/GSK-1265744 is an integrase inhibitor, similar to dolutegravir. Results from two studies presented at ICAAC 2012 found that 744 had high potency and an exceedingly long half-life. When given orally at once-daily doses of 30 mg, patients showed a median 2.6 log reduction in viral load. More impressive, when using nanotechnology to formulate 744 to be injected subcutaneously or intramuscularly, a single dose showed a half-life between 21 and 50 days. Remarkably, after a single dose, patients still had detectable levels of 744 up to 48 weeks after injection.

Similar to dolutegravir, 744 seems to have a high barrier to drug resistance. According to Gulick, using site-directed molecular clones (molecules created with specific mutations) associated with integrase inhibitor resistance, these mutations showed high levels of resistance to raltegravir and elvitegravir, but remained susceptible to 744 and dolutegravir.

In terms of safety, there were some injection site reactions and nodules associated with subcutaneous dosing. But conceivably, 744 could be taken as infrequently as every three months for treatment, or even as PrEP (pre-exposure prophylaxis). Research is ongoing.

HIV Entry Inhibitors

HIV entry inhibitors block HIV at the point at which they attach to CD4 cells. Many of the other classes of drugs, including protease inhibitors, NRTIs and NNRTIs, fight HIV after it has infected a CD4 cell.

Among the entry inhibitors, there are presently three sub-classes. We already have approved drugs in the first two sub-classes: CCR5 antagonists and fusion inhibitors. In the CCR5 antagonist sub-class, we already have maraviroc (Selzentry, Celsentri), and a new drug called cenicriviroc is being investigated. The fusion inhibitor class has long featured only enfuvirtide (Fuzeon), but a new drug called albuvirtide is being studied.

The third sub-class is an investigational one: CD4 attachment inhibitors. These new drugs are being developed to either bind at the location of HIV's gp120 protein (such as BMS-663068) or on the CD4 receptor itself (such as ibalizumab, a once- or twice-a-day drug that's still under investigation for both treatment and prevention).

Gulick offered a closer look at a few of these drugs in his overview.

Cenicriviroc (CVC)

Cenicriviroc is an investigational CCR5 antagonist. Not only does it antagonize CCR5 binding, it also antagonizes CCR2 binding. CCR2 is a receptor that sits on the surface of macrophages and may be involved in inflammation.

Cenicriviroc showed potent antiretroviral activity in a small study by Jacob Lalezari, M.D., and others. They followed treatment-experienced patients who had not been on treatment for at least six weeks, had a CD4+ cell count above 250 and a viral load above 5,000 copies/mL. They were randomized to receive either 25, 50, 75, 100 or 150 mg of once-daily cenicriviroc. At the highest doses, after 10 days, patients showed a 1.5 log reduction in viral load.

An update on the follow-up study was discussed by David Martin, M.D., at CROI 2012. In Martin et al's phase-2b study, they randomized 150 treatment-naive patients into three groups to receive tenofovir/emtricitabine with either cenicriviroc (at 100 or 200 mg) or efavirenz. Cenicriviroc was administered using a new, 50-mg formulation. In preliminary data from 18 patients, cenicriviroc was found to be well-absorbed and within the expected therapeutic range of potency. Further study will assess the safety, efficacy and effect of CCR2 inhibition on inflammatory biomarkers.

Albuvirtide

The only approved fusion inhibitor, enfuvirtide, offers a lot of activity against HIV, but the obvious downside is that it requires twice-daily injections. Albuvirtide, on the other hand, is an investigational fusion inhibitor that when given intravenously has a long average half-life of 11 days, warranting weekly dosing.

Albuvirtide has a similar design to enfuvirtide. It is a peptide that is an analogue of gp41, one of the envelope proteins on HIV's surface, and thereby blocks HIV through CD4 membrane fusion.

Two proof-of-concept studies by Dong Xie and others were presented at ICAAC 2012. In the first study, albuvirtide was given to 54 treatment-naive patients in a single dose; doses ranged from 20 to 640 mg. They found that albuvirtide's half-life ranged between 10 and 13 days, and that the drug suppressed plasma viremia for between 6 and 10 days. Albuvirtide was generally well tolerated, with no injection-site reactions and no serious adverse events.

In the second study, albuvirtide was given to 12 treatment-naive patients in multiple doses of either 160 or 320 mg. Doses were given on days 1, 2, 3, 8 and 15. The participants averaged viral load decreases of 0.68 log copies/mL (at 160 mg) and 1.05 log copies/mL (at 320 mg). Similar to the first study, there were no injection-site reactions and no serious adverse events. No anti-albuvirtide antibodies were detected in patients for up to 42 days.

BMS-663068 (a.k.a. BMS-068)

BMS-068 is an HIV attachment inhibitor. It is an oral prodrug that breaks down into the active compound BMS-626529 (a.k.a. BMS-529). It inhibits CD4 binding by specifically binding to gp120, one of HIV's envelope proteins that binds to CD4 cells. Gulick stated BMS-068 could be taken once or twice a day without boosting, but noted, "There is decreased baseline susceptibility in some patients due to envelope polymorphisms."

BMS-068 taken over 8 days with or without ritonavir (Norvir) resulted in substantial declines in plasma HIV RNA levels and was generally well tolerated, according to a study by Richard Nettles, M.D., and others. The study followed 50 patients with a CD4+ cell count above 200 cells/mL and a viral load above 5,000 copies/mL. They were either treatment-naive or not taking any treatment. The median change in viral load ranged from a 1.21 to a 1.73 log reduction, demonstrating that CD4 attachment inhibition can be quite potent and effective.

In terms of resistance, a study presented by Neelanjana Ray, M.D., at CROI 2012, found little resistance after the eight days of monotherapy. Ray and his team analyzed the changes in phenotypic susceptibility and known attachment inhibitor resistance substitutions that may have occurred during the Nettles study. Of the 48 patients that completed the study, 42 had at least a 1 log drop in viral load, showing that BMS-068 was effective. However, the other six (about 12%) had no virologic response, even though their baseline IC 50 (a measure of the effectiveness of a compound in inhibiting a biochemical function) levels were quite high.

"When they took a close look and they sequenced gp160, which is broken down into gp120, they showed that a mutation (M426L) was associated with resistance," explained Gulick. "You can see that in patients with virologic response, very few (only 6%) have this mutation, whereas in those without virologic response, 5 of the 6 had this mutation. So it looks like this will be important in screening for activity of this compound as its development moves forward."

New Fixed-Dose Combination and Antiretroviral Formulations

Gulick pointed out three one-pill, once-a-day formulations being developed:

GS-7340/FTC/elvitegravir/cobicistat.

GS-7340/FTC/darunavir/cobicistat.

Abacavir/lamivudine (3TC, Epivir)/dolutegravir.

In addition, three other in-development formulations that Gulick mentioned were:

Lopinavir/ritonavir/lamivudine.

Atazanavir (Reyataz)/cobicistat and darunavir/cobicistat (both in clinical trials).

Rilpivirine long-acting (RPV-LA).

Regarding the last drug in those lists, a small pilot study presented at CROI 2012 found that RPV-LA could potentially be given once a month in its long-acting nano-formation. It would likely need to be paired with other drugs, but research is ongoing for its use in treatment and prevention.