December 17, 2001
Tenofovir, a drug recently approved for sale in the U.S. under the brand name Viread, had two large studies which led to its approval. One of these studies was presented here. Like the companion trial, it was a study of those on antivirals who experienced ongoing viral replication despite their regimen. In this study, either tenofovir or a placebo was added for 24 weeks, and then at week 24, the placebo group also received tenofovir.
The study was a large one, with about 550 participants. They were eligible to join if, despite being on antivirals, their viral load was between 400 and 10,000 copies. Tenofovir belongs to a new class of antivirals chemically related to the nucleoside reverse transcriptase inhibitors (also called NRTIs, including AZT, ddI, and 3TC). This study was done in those patients who had some degree of viral resistance to this family of antivirals, and helped to address what tenofovir can do for them.
In terms of safety, one of the more impressive aspects of this agent is the lack of toxicity which can lead to drug discontinuation. While three percent of the people on tenofovir stopped it due to some toxicity, three percent on the placebo stopped as well.
This impressive outcome was mirrored in the lack of significant clinical side effects and the lack of major lab toxicity noted from tenofovir. Unlike with other drugs in the same chemical class as tenofovir, which have a history of kidney function disturbances, no one in these studies had a creatinine rise over 2.0, and only one percent had a rise above their baseline to a level below 2.0, which confirms the safely of this agent. Also of note is that about four percent had a drop in serum phosphate, although in the majority of cases it normalized without any specific intervention.
In terms of benefits, the average viral load drop was 0.6 log. Furthermore, about thirty percent went to below 400 copies, and about twenty percent went below 50 copies just by adding this single medication. There was a 25-cell increase in the CD4 cell counts. This average viral load decline did vary by degree of resistance to tenofovir, as presented from data from this and other studies combined. For example, patients who only had a 184V mutation noted a one log drop in viral load, while those with many more resistance mutations had a blunted drop.
Fortunately, only a few patients have a mutation at position 65, which appears to completely blunt the impact of tenofovir. In all, this 0.6 log drop is accompanied by typical patterns of resistance as noted in those with less than fully suppressive mutations while taking NRTIs.
These data together confirm the results from another study that compared the now approved 300mg-daily dose to other lower doses. It clearly demonstrates that tenofovir can play an important role in the treatment of many people who are on regimens that are not fully successful, as the simple addition of tenofovir can help to lower viral load in a way that is durable to at least 24 weeks.
The uses of tenofovir, however, can be extended beyond simply adding it as a single new agent to a failing regimen. This study demonstrates the excellent safety record of this drug, a key finding that will be monitored over time to determine whether its initial safety pattern can be maintained for years rather than months.
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