The extensive cross-resistance that exists between currently approved protease inhibitors (PI) concerns a large number of HIV-1-infected patients. Despite the pharmaco-enhancement of PI plasma exposure with ritonavir and consequently the increase in the resistance level, the efficacy of boosted PI combinations remains greatly influenced by the extent of baseline PI resistance mutations . The accumulation of resistance mutations on the protease gene limits the sequential use of these agents .
Darunavir (TMC 114) is an investigational PI now available for heavily treatment-experienced patients through an expanded access programme. In-vitro data suggest that darunavir has extremely potent antiretroviral activity (EC50 1–5 nmol) and is able to maintain this activity against HIV variants that are highly cross-resistant to current PI. The in-vitro selection experiments starting from wild-type HIV-1 also showed that darunavir has a very high genetic barrier to the development of resistance . This potent activity against PI-resistant HIV indicates that this new antiretroviral agent could be particularly useful in treating patients failing a PI-containing regimen . In Power randomized clinical studies [5,6], many PI-experienced subjects receiving darunavir/ritonavir (600/100 mg twice a day) achieved significantly greater viral load declines at 24 weeks than comparative PI. In a recently reported resistance substudy , the specific baseline PI mutations associated with reduced response to darunavir/ritonavir 600/100 mg were V11I, V32I, L33F, I47V, I50V, I54L/M, G73S, L76V, I84V, and L89V. That study also described the emergence of mutations V32I, L33F, I47V, I54L, and L89V in more than 10% of non-responder patients. It is therefore important to collect, in clinical practice, complementary resistance data that impact darunavir/ritonavir virological response. Factors associated with virological outcome to darunavir/ritonavir, particularly resistance mutations profile, in eight heavily experienced patients harbouring multidrug resistance viruses, were studied.
Since February 2006, the French Drug Agency has authorized the prescription of darunavir/ritonavir to HIV-1-infected patients with no available treatment option, through a temporary authorization for use. Patients who received a darunavir/ritonavir (600/100 mg twice a day)-containing regimen as salvage therapy for at least 3 months during precommercialization use were studied. Inclusion criteria were: (i) multiple failures to all current classes of antiretroviral agents, including ritonavir-boosted PI; (ii) plasma HIV-1 RNA greater than 10 000 copies/ml; and (iii) the presence of protease mutations associated with resistance to available approved PI. The baseline characteristics including CD4 cell count, HIV-1 viral load and genotypic resistance test, therapeutic history, and genotypic sensitive score (GSS) of associated antiretroviral agents were collected. Plasma darunavir trough concentrations were determined using a high-performance liquid chromatography-coupled fluorimetric detector (limit of quantitation 5 ng/ml).
Eight patients with severe immunosuppression [median (range) nadir of CD4 cell count 45 cells/μl (1–190)] began rescue therapy with darunavir/ritonavir associated with at least two reverse transcriptase inhibitors and enfuvirtide (Fig. 1). They had received a median of eight antiretroviral regimens (four to 12), with a mean of 6.2 nucleoside reverse transcriptase inhibitors, 1.4 non-nucleoside reverse transcriptase inhibitors and 4.7 PI (including amprenavir or fosamprenavir in all cases). Four patients were pretreated with tipranavir, four with enfuvirtide and three with both drugs. The median baseline CD4 cell count and plasma HIV-1 RNA were 19 cells/μl (4–550) and 5.2 log10 copies/ml (4.4–6), respectively. A baseline genotypic resistance test reported the median number of six nucleoside reverse transcriptase inhibitors (3–7), one non-nucleoside reverse transcriptase inhibitor (0–2) and 13 PI (10–15)-associated resistance mutations (Fig. 1). Among the darunavir-associated mutations, three patients harboured one, four patients harboured two and one patient harboured three mutations. According to the ANRS algorithm, all but one patient harboured darunavir/ritonavir-susceptible viruses. In contrast, darunavir resistance was intermediate for all viruses with regard to the Stanford algorithm. GSS was calculated with the baseline genotype and enfuvirtide was considered active only in non-pre-exposed patients. The median GSS was 1.25, ranging from zero to three active drugs associated with darunavir/ritonavir in the salvage regimen. The median darunavir trough concentration was 2612 ng/ml (1173–6932).
The median decrease in HIV-1 viral load from baseline was −2.8 log10 copies/ml at month 1, at month 3, and at month 6, respectively (Fig. 1). A plasma HIV-1 RNA of less than 400 and less than 50 copies/ml were observed at month 1 for six out of eight and two out of eight patients, at month 3 for six out of eight and five out of eight patients, and at month 6 for six out of eight and five out of eight patients, respectively. The median increase in CD4 cell counts from baseline was +57, +61, and +113 cells/μl at months 1, 3 and 6, respectively. Virological failure was reported in two patients (Fig. 1). In patient 4, the viral load was never suppressed and resistance analysis at month 6 showed new mutations 13V, 32I, 33F, 46I/L, 47I/V and 84V, a shift at mutated codons 10F to I, 54V to L, 73A to S and a loss of 36I and 50V. In patient 6, viral rebound was observed at month 6 with the selection of I50V to the same backbone protease resistance profile.
Risk factors leading to virological failure, such as multiple therapeutic regimens, tipranavir/enfuvirtide pre-exposure, a high degree of baseline drug resistance, low number of active drugs (low GSS), high baseline viral load and low baseline CD4 cell count were similar between the two darunavir regimen-failing patients and the six others. In spite of the high number of PI mutations and extensive amprenavir resistance according to ANRS and Stanford (HIV database) algorithms for all patients (Fig. 1), it seems important to emphasize that specific amprenavir resistance profiles, such as I50V and V32I + I47V, were detected at baseline only in the two darunavir-failing patients. Moreover, the emergence of these two key resistance profiles occurred at virological failure in these two patients (Fig. 1). Interestingly, darunavir trough plasma concentrations were below the target value for resistant viruses of 2000 ng/ml  for the two failing patients (1173 and 1453 ng/ml) compared with the mean (3776 ng/ml) for the six others.
As darunavir was chemically similar to amprenavir , this result could be expected. In vitro data, however, showed that most clinical isolates resistant to amprenavir exhibited susceptibility to darunavir [3,10]. No impact of the previous use of fosamprenavir, tipranavir and lopinavir was observed on the darunavir/ritonavir virological response in Power studies . These findings did, however, not take into account key amprenavir-selected mutations (I50V and V32I + I47V) , but only overall amprenavir susceptibility. As shown with other PI, the relative weight of resistance mutations may be determined to predict the efficacy of darunavir/ritonavir in experienced patients. This result suggests that antiretroviral treatment with darunavir in patients with key amprenavir resistance should be considered with caution. A further analysis of large clinical databases could assess the impact of the specific amprenavir resistance profile on the response to darunavir.
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