453   Absence of Resistance to Kaletra (ABT-378/r) Observed through 48 Weeks of Therapy in Antiretroviral-Naive Subjects.

B. Bernstein*, J. Moseley, D. Kempf, M. King, K. Gu, E. Bauer, and E. Sun.
Abbott Labs, Abbott Park, IL.

Background:Study M98-863 is a randomized, double-blind, phase III study of 653 ARV-naive subjects treated with either ABT-378/r (n=326) or NFV (n=327), in combination with d4T and 3TC. In an on-treatment analysis, 93% of ABT-378/r-treated subjects and 82% of NFV-treated subjects achieved VL < 400 copies/mL at wk 48 ( P < 0.001).

Methods:In order to gain insight into the development of resistance to ABT-378 the plasma viral isolates from all subjects with HIV RNA >400 copies/mL on treatment at either wk 24 or wk 48 (ABT-378/r, n=42; NFV, n=78) were examined for evidence of resistance. Resistance for NFV was defined by the presence of the D30N or L90M mutation, and ABT-378 resistance was defined as any primary or active site mutation. Genotype (population sequencing) and phenotype (PhenoSense^TM) were performed by virology, Inc.

Results:The data from wk 24 and wk 48 combined are presented in the table below:

Of the 21 subjects who had resistance to NFV, 7 (33%) had the L90M mutation.

Conclusions:Of subjects with a viral load >400 copies/mL at wk 24 or wk 48, 32% of the NFV-treated subjects vs. 0% of the Kaletra^TM-treated subjects displayed protease inhibitor resistance.