Importantly, the clinical research supports once daily dosing of Ziagen (abacavir) and Epivir. One recent clinical study of naive patients (patients not previously treated) evaluated once daily administrations of both Epivir and Ziagen combined with Sustiva (efavirenz). The Ziagen/Epivir once daily arm was randomly compared to Ziagen administered twice daily for 48 weeks. (ZODIAC – CNA30021, 43 rd ICAAC, Sept 2003). Both groups of patients had identical antiviral benefit with no increased side effects observed in the once-daily group.
In contrast, preliminary results from a recent trial (study 934) may have also illustrated how PK and tolerability can impact on treatment success. This study compared the components of Truvada, Viread + Epivir once daily, to treatment with twice daily Combivir or AZT + Epivir. AZT’s intracellular half life is very short and only 3 hours; its plasma half life is 1.1 hour. Preliminary data at 24 weeks showed superiority in the Viread/Emtriva once daily vs. Combivir (88% vs. 80% had viral loads <400 copies). All study patients in both arms were also treated with Sustiva.
Once A Day Keeps the Doctor Away
Adherence is another way of describing the habit of taking all medication doses as prescribed. Good adherence is required to achieve the best therapeutic results. However, adherence can be influenced by life style, dosing and side effects. Treatments need to correspond to individual patient needs. Thus, new formulations like Truvada and Epzicom are indeed a welcome addition to our treatment landscape and should add to overall convenience for both new patients starting on treatment as well as experienced patients. In other words, one hopes and expects that these new additions will improve adherence for many individuals.
Moreover, a clinical study demonstrated once-daily dosing of HAART was better than multi-dosed treatment and was presented at the 10 th CROI in 2003 (Molina JM et al). Individuals who had suppressed viral load levels were switched randomly to a new once-daily regimen of emtricitabine (Emtriva), didanosine (ddI-EC), and efavirenz (Sustiva) versus continuation of their old multi-dose protease inhibitor regimens. There was a significant difference favoring the once-daily arm, as 95% vs. 87%, once daily vs protease inhibitor arms, had undetectable viral loads below 50 copies. T he once-daily regimen resulted in improvements in adherence. As more data from large randomized trials become available, once-daily HAART may demonstrate better durability in treatment throughout.
Combining these agents into one pill provide the practicality of lower bill burden and ease of administration. In contrast, pushing individuals to once-daily drug regimens may have other implications. A gents that do not provide full coverage for 24 hours and missing doses while on a once-daily HAART can potentially be a danger for subtherapeutic blood levels during delayed dosing. As an example, in twice daily regimens, one missed dose a week translates to one out of 14 doses or 8% missed. One missed dose of once-a–day regimen equals missed for a complete 24 hour period or 15% missed. In this situation, more time without antiviral coverage means suboptimal viral suppression. This usually results in lowered antiretroviral plasma (blood) levels leading to development of resistant strains.
Despite the missed dosing drawback, it is generally agreed that once-daily dosing is more practical and patient friendly; ultimately it should improve durability through better adherence. Because there are other factors that influence adherence, seasoned HIV physicians understand that there is no magical answer for all patients.
Resistance and Controversies Regarding Drug Sequencing
There exists much controversy regarding the sequencing of these drugs. When we refer to sequencing we are describing a chess game. When making each move on the chess board, one needs to keep in mind your partners next move against you, and your own next move following. Likewise, choosing an antiviral regimen mandates an awareness of potential resistance later and a plan for the next treatment options ahead. Resistance and sequencing is complicated and there is much we are still learning. Thus starting a regimen containing Epzicom or Truvada, planning for the next treatment options down the road is important.
Various mutations are associated with resistance to specific antiviral agents. For example, mutations such as the M184V is associated with Epivir resistance and K65R can be associated with Ziagen, ddI and /or Viread. If Epivir resistance develops during treatment with Epzicom (M184V mutation), then later treatment with Viread continues to remain a treatment option. However, if Epzicom selects for the L74V mutant, a recent study showed this to potentially result in the eventual emergence of the K65R which can potentially block an option for later treatment with Viread.
During phase 2 and 3 trials (studies 902 & 907), Viread was given to highly treatment experienced patients. The infrequent presence of K65R was associated with only low grade resistance to Viread. The few patients during these trials who had this mutation also had a history of Ziagen or ddI treatment (previously), which was implicated as the cause of the K65R. Moreover, the K65R mutation is associated with a low-grade resistance to ddI and Ziagen, and moderate resistance to Epivir. In a real world clinic, resistance to Viread occurs relatively infrequently and patients who have the K65R often respond to Viread treatment (and is often sensitive by phenotypic testing).
To make things more complicated, if a patient takes Truvada and develops both K65R and M184V, there is the potential to eliminate later options for treatment with Ziagen and Epzicom. However, if one developed resistance with Combivir or AZT, the mutations are generally of the TAMS variety or thymidine associated mutations. This also blocks one’s potential options with Viread later on as well as other agents.
Thus one can see how complicated drug sequencing becomes, especially when newer treatments come to market. We usually don’t have all the answers at first.
Summary
The arrival of the two new formulations, combinations of two NRTI’s each, have made it simpler for patients. There are still some unanswered questions and clinicians debate about which drug combinations of nukes, non-nukes, protease inhibitors and boosted protease inhibitors are best suited for starting patients when beginning therapy. Many patient specific particulars need to be considered and many details such as pharmacokinetics, resistance and sequencing all need to be measured. The availability of both Epzicom and Truvada are another step in the right direction towards improving HIV treatment. Both can be combined with other single dosed antivirals to construct true once daily regimens. They can also be combined with other dosing regimens to reduce pill burden. Furthermore, a recent press release announced that Bristol-Myers-Squibb, Gilead Sciences and Merck have announced plans to develop a fixed-dose combination pill of three antivirals.
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