Background: The NEFA study is the first randomized study aiming to compare nevirapine (NVP), efavirenz (EFV), or abacavir (ABC) as substitutes for protease inhibitors (PI) in a large group of HIV-1-infected patients (pts) successfully treated with PI-containing antiretroviral regimens.

Objective: To evaluate genotype and phenotype resistance testing among pts who have experienced virological failure under one of the 3 simplification arms.

Methods: HIV isolated from plasma from pts with virological failure defined as 2 consecutive determinations of HIV-1 RNA > 200 copies/ml was analyzed for phenotypic susceptibility by VIRCO Antivirogram. HIV-1 mutations were detected using the ABI ViroSeq HIV-1 Genotyping System (Applied Biosystems).

Results: Forty (40; 8.7%) of 460 pts eligible for the study experienced virological failure after 18 months of follow-up.

 

Table 1. Resistance testing among patients with virological failure while on study medication

	N	NA	Total	NAm/BDL	ABC (%)	NVP(%)	EFV(%)
Total patients	460				149	155	156
Genotype	40	7	33	10 (30%)*	15 (10.1)	5 (3.2)	3 (1.9)
Phenotype	40	12	28	10 (36%)	12 (8.1)	4 (2.6)	2 (1.3)

NA: not available; NAm: not amplified. BDL: recombinant virus yield below the detection limit

*HIV-1 RNA less than 1.000 copies/ml in 9/10

 

A higher proportion of pts in ABC than NVP and EFV arms showed resistance to the study drugs. Moreover, a much higher number of resistance mutations to one or more of the backbone nucleosides contained in the failing regimen were observed in the ABC arm—M41L, K65R, D67N, T69N, K70R, L74I/V, M184V, L210W, R211K, T215F/Y, K219E/Q- than EFV –M41L, A62V, M184V, R211K, T215Y- and the NVP arm—M41L, K70R, M184V, T215Y-.

 

Table 2. Genotypic and phenotypic results

Test		AZT	D4T	ABC	ddI	3TC	TDF	NVP	EFV	NFV	SQV	IDV	RTV	AMP	LPV
GENO n = 23	S	8	14	1	4	4	14	15	15	15	21	20	20	21	22
	PR	3	6	12	17	1	7	--	--	--	--	1	1	1	--
	R	12	3	10	2	18	2	8	8	8	2	2	2	1	1
PHENO n = 18	S	12	15	14	18	2	5	13	13	13	18	16	16	17	17
	R	6	2	4	--	16	1	5	5	5	--	2	2	1	1

S = sensitive; PR = possible resistance; R = resistance.

 

Overall, there was a good correlation among genotype and phenotype resistance testing, the latter adding valuable information-fold change in IC50- mainly among those NRTIs (d4T, ABC, ddI) showing possible resistance based on genotypic interpretation.

Conclusions: Cross-resistance involving nucleoside analogues might explain the higher risk of virological failure in pts switched to ABC-containing HAART. Phenotypic resistance testing may be helpful to better ascertain those genotypic results of difficult interpretation.