Switching in the absence of failure
Switching or substitution of a single drug in a regimen may be indicated in the following instances;
- To reduce cost of the regimen
- To simplify the regimen so as to improve adherence
- To manage toxicities
Before substitution of a single Drug it is important to get a PVL estimation to ensure that the regimen is effective. If not effective, resistance may be expected and substitution of a single Drug is not recommended.
The commonest example of switching for avoiding long-term toxicity would be between d4T and ZDV to ABC or TDF.
Managing treatment interruptions
Often patients on stable ART miss doses or discontinue drugs. A careful adherence history needs to be taken to find out exactly how the doses were missed. Resistance development is more likely in a patient who takes Drugs intermittently than in someone who discontinues therapy all together. Resistance may also be more likely when therapy interruptions occur early after treatment initiation rather than later on, particularly after the PVL has been undetectable. The decision to reinitiate the first-line regimen or change to a second-line regimen should be considered accordingly. If the decision to reinitiate therapy is taken, VL estimation at the end of 3-6 months is recommended to assess effectiveness of therapy. Nevirapine has to be re-initiated in a lead-in dose if the interruption has been for more than 7 days.