Untimed Efavirenz Drug Levels After Switching From Brand to Generic Formulations: A Short Communication

Background: In British Columbia, antiretrovirals are distributed at no cost to patients via a publicly funded program, using generic formulations if available. A generic efavirenz-emtricitabine-tenofovir DF (EFV-FTC-TDF) combination pill became available in April 2018. The authors compared EFV untimed drug levels in subjects switching from brand to generic EFV-FTC-TDF. Methods: Archived plasma HIV viral load samples were identified for consenting participants who switched from brand to generic EFV-FTC-TDF; 3 preswitch and 2-3 postswitch samples, collected >= 1 month apart were assessed for each subject. "Untimed" EFV concentrations with unknown dosing and collection time were assessed using a validated liquid chromatography-tandem mass spectrometry method. Participants' mean, minimum, and maximum EFV levels were compared using the Wilcoxon signed rank test. Participants with EFV levels in the range associated with lower risks of virologic failure and central nervous system toxicity (1000-4000 ng/mL), preswitch and postswitch, were enumerated. Results: EFV levels were assessed in 297 preswitch and 249 postswitch samples from 99 participants, having exposure to brand and generic EFV for a median of 103 (Q1-Q3: 87-116) and 10.3 (Q1-Q3: 8.9-11.7) months, respectively. The final brand sample was collected at a median of 98 days preswitch; the first generic sample was collected at a median of 133 days postswitch. No significant differences were observed in participant mean EFV levels before (median 1968 ng/mL; Q1-Q3: 1534-2878 ng/mL) and after (median 1987 ng/mL; Q1-Q3: 1521-2834 ng/mL) switch (P = 0.85). Eighty participants had mean EFV levels within the 1000-4000 ng/mL range on the brand drug, of which 74 remained within this range postswitch. Conclusions: There were no statistically significant differences between untimed EFV levels in patients switching from the brand to generic EFV combination pill. Given the long elimination half-life of EFV, untimed drug levels may be a convenient way to estimate product bioequivalence.