High virological failure rate in HIV patients after switching to a regimen with two nucleoside reverse transcriptase inhibitors plus tenofovir

Background: Regimens with two nucleoside analogue reverse transcriptase inhibitors (NRTI) plus tenofovir DF have been associated with a high failure rate when administered as first line therapy. Little is known about patients with undetectable viral loads who are switched to these regimens.

Methods: A post-hoc review of the virological outcomes at 24 weeks of patients who switched from a successful (< 50 copies/ml) highly active antiretroviral therapy regimen to a tenofovir plus two NRTI combination.

Results: Fifty-five patients started a two NRTI plus tenofovir regimen mostly because of previous toxicity/intolerance of the original drugs (74%). After 24 weeks, only 17 patients (31%) remained virologically suppressed. Patients with a regimen including a didanosine plus tenofovir-based regimen had significantly poorer outcomes than those on other combinations (success rate 5 versus 47.1%, P = 0.001). In contrast, patients on a regimen including zidovudine plus tenofovir showed a trend towards a better outcome (75 versus 27%, P = 0.083). Multivariate analysis confirmed the combination of didanosine plus tenofovir as the only variable associated with a higher rate of failure (odds ratio 17.7; 95% confidence interval 2.1-147; P = 0.007). Patients with previous reverse transcriptase mutations presented virological failure in all cases. At failure a new pattern, including the K65R mutation with M184V or thymidine analogue mutations, was observed.

Conclusions: Even in patients with suppressed viraemia, a two NRTI plus tenofovir regimen is associated with a high virological failure rate, but significant variations are found depending on the nucleosides included.

Introduction

Since the availability of tenofovir DF, new possibilities of combining the drug with nucleoside reverse transcriptase inhibitors (NRTI) have emerged ^[1]. Unfortunately, clinical trials with combinations that included abacavir plus lamivudine plus tenofovir ^[2-4] or didanosine plus lamivudine plus tenofovir ^[5] have shown unexpectedly high rates of virological failure. A high virological failure rate has recently been reported in patients who were switched to a regimen containing abacavir, lamivudine and tenofovir, in the setting of virological suppression ^[6]. It is thus recommended that this combination be avoided in most, if not all, patients.

At present there is no information on other combinations of two NRTI and tenofovir. For this reason, we investigated treatment response in previously suppressed patients who were switched to a two NRTI plus tenofovir regimen, in order to extend previous observations, and to assess the response rate with combinations of NRTI other than lamivudine/didanosine and lamivudine/abacavir.

Patients and methods

We analysed all patients who had been prescribed two NRTI plus tenofovir between October 1999 and September 2003 and who had a follow-up of at least 24 weeks. For the purposes of this analysis, we included only patients who had been virologically suppressed (viral load below the limit of detection with branched DNA version 3.0, Bayer Diagnostics (Chiron, Emeryville, California, USA; limit of detection 50 copies/ml), detection limit of 50 copies/ml) and on a stable regimen during the 24 weeks before switching to two NRTI plus tenofovir. Clinical, virological and immunological evaluations were carried out every 3 months. The main endpoint was maximal viral suppression (HIV-RNA level < 50 copies/ml) at week 24. HIV-RNA level considered failures were confirmed with a new viral load. Loss to follow-up and any change of current antiviral therapy for any reason were considered failures. Adherence was assessed by means of a questionnaire in the outpatients clinic and the pharmacy department records.

We were able to analyse some historical and at-failure resistance genotypes, focusing on the reverse transcriptase (RT) pattern. The genotypes evaluated were those performed for routine clinical practice, and were not selected with any systematic criteria. For the present analysis, we identified all the mutations considered as RT significant mutations, including six thymidine analogue mutations (TAM) (M41L, D67N, K70R, L210W, T215A/C/D/E/G/H/I/L/N/S/V, K219Q/E), M184V, K65R, L74V, and M151Q.

Continuous variables were compared using Student's t-test for variables with normal distribution, or Mann-Whitney or Wilcoxon tests for continuous variables with unequal variances or non-normal distribution. Categorical data were assessed using the chi square test. Univariate and multivariate analyses, with viral load as the dependent variable, were performed to assess factors influencing virological outcome. Data were censored in December 2003.

Results

Fifty-five patients received a two NRTI plus tenofovir-based regimen. At baseline, all the patients had a CD4 cell count above 300 cells/mm³ and an HIV-RNA level lower than 50 copies/ml for more than 24 weeks. Only 34.5% had not received previous suboptimal NRTI or had not failed with a lamivudine-based regimen. Treatment before the switch included a three NRTI-based regimen in 32 patients (58%), a protease inhibitor-based regimen in 12 patients (22%) and a non-nucleoside reverse transcriptase inhibitor-based regimen in 11 patients (20%). The main reason for switching to the studied regimen was toxicity (74%) (Table 1). After 24 weeks, only 17 patients (31%) remained suppressed (HIV-RNA level < 50 copies/ml) with the initial regimen. The other 38 patients considered as failures were 26 patients (47%) with an HIV-RNA level greater than 50 copies/ml in two determinations, 10 patients (18%) who had changed as a result of toxicity or intolerance to one or more drugs of the regimen, and two patients (4%) who were lost to follow-up at 24 weeks.

Table 1

Although the figures are too small to draw any definite conclusions, the virological rate of suppression varied depending on the two NRTI combinations associated with tenofovir: one patient receiving zidovudine plus abacavir, two out of three (67%) on zidovudine plus lamivudine, three out of six (50%) on stavudine plus lamivudine, 10 out of 23 (43%) on abacavir plus lamivudine, one out of 13 (8%) on didanosine plus lamivudine, and 0% in patients treated with didanosine plus abacavir (seven patients), didanosine plus stavudine (one patient) and stavudine plus abacavir (one patient). When compared with other regimens, a regimen that included didanosine had a significantly poorer virological success rate [1/21 (5%) versus 16/34 (47.1%), P = 0.001], whereas those that included zidovudine presented a trend towards a better outcome [3/4 (75%) versus 14/51 (27%), P = 0.083]. We have focused on patients receiving a combination that included didanosine plus tenofovir. All patients had the didanosine dose adjusted to weight (250 mg in patients weighing 60 kg, 200 mg in patients weighing < 60 kg) since September 2002. Only two patients received the high dose of didanosine for more than 3 months of the 21 patients in this group; 19 had an adherence to therapy greater than 95%. Causes of failure before 24 weeks of therapy included virological failure in 18, toxicity in one patient who developed lactic acidosis while receiving concomitant ribavirine, and loss of follow-up in one patient.

Multivariate analysis confirmed the didanosine plus tenofovir combination as the only variable related to a higher probability of failure (odds ratio 17.7; 95% confidence interval 2.1-147; P = 0.007) after adjusting for previous AIDS diagnosis, adherence, baseline CD4 cell count, or having taken another NRTI-tenofovir-based regimen. Unexpectedly, no significant differences in virological success were observed among patients with previous suboptimal NRTI regimens or among those who failed with a lamivudine-based regimen or the combination of both variables (31 versus 32%, P = 0.58).

A genotypic resistance test, performed at any time before switching to the current tenofovir-based regimen, was available for 10 patients. As previously stated, resistance testing was not performed under any systematic criteria, only the caring physician indication. Six patients had no significant mutations in the RT, whereas four isolates exhibited TAM, two of them with M184V (Table 1). Patients with baseline mutations presented a trend towards a worse virological outcome at week 24. No patient achieved HIV-RNA levels less than 50 copies/ml (0%) compared with four out of six patients (66%) without mutations (P = 0.076).

At failure, eight patients had genotypic resistance testing performed. TAM had been selected in five patients, M184V in six and K65R in four. Resistance patterns at failure included: TAM plus M184V (three patients), K219Q/N plus K65R (two patients), M184V plus K65R (two patients) and M184V alone (one patient).

Discussion

This work highlights the unexpected short-term failure rate observed with triple drug therapies containing two NRTI plus tenofovir in previously treated and virologically suppressed patients. These findings are in agreement with the results of recent reports in naive patients, when combinations including abacavir, lamivudine and tenofovir ^[2-4] or didanosine, lamivudine, and tenofovir ^[5] were used. Simplification strategies to regimens containing two NRTI plus efavirenz, nevirapine or abacavir have yielded success rates above 70% at week 48 ^[7], whereas the overall success of two NRTI plus tenofovir in our study was approximately one third of patients, as recently reported in other observational studies ^[6,8].

The reasons that may help explain these poor results include recently rejected pharmacokinetic interactions ^[9,10], the potency of the combinations ^[11,12], and a high selective pressure for resistant isolates ^[2-5].

It is interesting to note the different response rates observed among the different NRTI combinations. According to our results, regimens containing zidovudine and stavudine with lamivudine presented the highest rate of success. A good explanation could be the negative interaction observed between the most frequent mutations selected the TAM and M184V or K65R, confirmed by in-vitro as well as by clinical studies ^[13,14]. Moreover, molecular antagonism is less likely because they are pyrimidinic base analogues. Our results agree with the recent observational analysis of a German cohort ^[8] that included all experienced patients who were being treated with a tenofovir-based regimen, abacavir and didanosine selected the K65R mutation in two NRTI plus tenofovir combinations.

Genotypic analysis in a small subset of our patients showed that the presence of previous RT mutations at any time in the past precluded any possibility of virological response in patients receiving two NRTI plus tenofovir. The patients with paired tests showed that some patients preserved TAM and the M184V mutation, whereas others selected for the K65R plus M184V, or the K65R with K219Q/N mutations, a pattern similar to that observed in naive patients. No patient presented a combination of K65R, M184V and any T215 variant, which was also observed in the German cohort study ^[8].

The results of our retrospective non-randomized analysis have to be confirmed with more powered studies, but these are the first reported outcomes for some two NRTI plus tenofovir combinations in virologically suppressed patients.

In conclusion, a high rate of virological failure is observed when a two NRTI plus tenofovir regimen is administered to patients who are virologically suppressed, irrespective of exposure to previous suboptimal regimens. Different NRTI combinations present different rates of failure, with the highest rate associated with combinations including didanosine, and the lowest with zidovudine-based regimens.

Acknowledgements

Other contributors to the study in the laboratory and in the clinical setting were Isabel García-Arata, María Pumares, Carmen Quereda, Paloma Martí-Belda, Cristina Morante, Vicente Muñoz, Enrique Navas and Antonia San Frutos.

Sponsorship: This work was partly supported by FIPSE (grant 2162/00); and Red Temática Cooperativa de Investigación en SIDA (red de grupos 173) del FIS.