Rates of antiretroviral resistance among HIV-infected patients with and without a history of injection drug use

Background: There exist concerns regarding the potential for elevated rates of antiretroviral resistance among HIV-infected injection drug users (IDUs) prescribed highly active antiretroviral therapy (HAART), however, no population-based study has examined if IDUs have elevated rates of antiretroviral resistance in comparison to non-IDUs.

Objective: To evaluate the time to the development of antiretroviral resistance among antiretroviral-naive patients with and without a history of injection drug use.

Methods: In British Columbia there is a province-wide HIV/AIDS treatment program that provides antiretrovirals free of charge. We examined all antiretroviral-naive patients initiating HAART between 1 August 1996 and 30 September 2000 and who were followed to 31 March 2002. The main outcome measure was the time to class-specific antiretroviral resistance. Cumulative antiretroviral resistance rates among IDUs and non-IDUs were evaluated using Kaplan-Meier methods and relative hazards were estimated using Cox regression.

Results: Overall, 1191 antiretroviral-naive patients initiated HAART during the study period. Resistance mutations were observed in 298 (25%) subjects during the first 30 months of HAART. In comparison with non-IDUs, the risk of protease inhibitor resistance [relative hazard (RH), 0.9; 95% confidence interval (CI), 0.5-1.6] and non-nucleoside reverse transcriptase inhibitor resistance (RH, 1.5; 95% CI, 1.0-2.2) were similar among IDUs, and there were no differences in the rates of resistance to the sub-classes of nucleoside reverse transcriptase inhibitors.

Conclusions: Resistance to all major classes of antiretrovirals were similar among IDUs and non-IDUs after 30 months of follow-up. These findings should help to allay fears that prescribing HAART to IDUs may result in elevated rates of resistance.

Introduction

The benefits of highly active antiretroviral therapy (HAART) in the management of HIV infection are well established ^[1]. Through the suppression of plasma HIV RNA, HAART has been shown to decrease morbidity and mortality among HIV-infected patients ^[2,3]. However, high levels of adherence to HAART are required to suppress plasma HIV RNA, and incomplete adherence has been associated with early emergence of antiretroviral resistance ^[4,5]. This is of particular clinical importance since there is substantial cross-resistance within antiretroviral drug classes ^[6], and there are a major challenges in treating patients with multi-drug resistant HIV ^[7,8].

As a result, there has been substantial public health concern regarding the transmission of antiretroviral-resistant HIV in the community ^[9-12], and studies have consistently identified physician concerns regarding antiretroviral resistance as the primary reason for withholding HAART from patients perceived to be at risk of non-adherence ^[13-15]. Previous studies have suggested that patients with a history of injection drug use (IDUs) may be disproportionately affected by this concern in many settings ^[13-16], which is a growing challenge given that injection drug users constitute a large and growing proportion of the HIV affected population globally ^[16-19].

However, no population-based study has examined if there are differential rates of antiretroviral resistance among patients with and without a history of injection drug use, and existing concerns regarding premature development of antiretroviral resistance among IDUs are not based on quantitative evidence ^[20]. Therefore, the present study was conducted to evaluate rates of antiretroviral resistance among patients with and without a history of injection drug use in a population-based HIV/AIDS treatment program that provides antiretroviral therapy free of charge.

Methods

Study population

The HAART Observational Medical Evaluation and Research (HOMER) study run through the British Columbia Centre for Excellence in HIV/AIDS Drug Treatment Program has been described in detail elsewhere ^[3,21]. Briefly, the Centre is the only free source of antiretroviral medications in the province of British Columbia and pharmaceutical sales suggest that < 1% of HIV-infected patients obtain antiretroviral drugs outside the program ^[22]. For all program participants, a complete prospective profile of HAART use is maintained and all plasma samples are stored for evaluation of plasma HIV RNA and the presence of antiretroviral-resistant HIV RNA.

In the present study, analyses were restricted to HIV-infected men and women who were antiretroviral naive and were first prescribed triple drug antiretroviral therapy between 1 August 1996 and 30 September 1999 and were followed for 30 months or up until 31 March 2002 if participant's follow-up was less than 30 months prior to this time. Study subjects were initially prescribed HAART with regimens including two nucleoside reverse transcriptase inhibitors (NRTI) and either a protease inhibitor (PI) or a non-nucleoside reverse transcriptase inhibitor (NNRTI) at the discretion of the enrolling physician. We were primarily interested in examining if patients with a history of injection drug use had different rates of resistance than patients without a history of injection drug use. Data on injection drug use obtained from the HOMER cohort is based on physician reports on the enrolment forms and on self-reports to the treatment program's annual participant survey ^[4,23]. As previously ^[4,23], to be conservative, we considered any positive report of this risk behavior at baseline or at any time during follow-up as indicative of having a history of injection drug use. Baseline characteristics of IDUs and non-IDUs were compared using Pearson's χ² test and the Wilcoxon rank sum test.

Drug resistance genotyping

The province's therapeutic guidelines recommended plasma HIV RNA levels be measured at baseline, after 1 month, and approximately quarterly thereafter ^[3]. This ensures that viral load monitoring is consistent across all physicians in the province and enables the Centre to ensure that treatments are in accordance with the therapeutic guidelines. Plasma HIV RNA was measured using the Roche Amplicor Monitor assay (Roche Molecular Systems, Mississauga, Ontario, Canada) as part of routine patient monitoring, and plasma samples were frozen for future use.

HIV drug resistance genotyping was attempted on

Equation (Uncited)

plasma samples with HIV RNA levels ≥ 1000 copies/ml collected in the entire 30 months following HAART initiation. Samples with HIV RNA < 1000 copies/ml were not systematically genotyped since genotyping does not yield consistent results on samples with low plasma HIV RNA, and were assumed to have no resistance mutations ^[24]. HIV RNA was extracted from plasma using the Qiagen viral RNA kit (Qiagen Inc., Mississauga, Ontario, Canada) with a BioRobot 9600/9604 or extracted manually using guanidinium-based buffer followed by isopropanol and ethanol washes. HIV protease (PR) and reverse transcriptase (RT) genes were amplified using nested RT-polymerase chain reaction (PCR), and sequenced in both the 5′ and 3′ directions. Results were reported as amino acid changes in the HIV PR and RT with respect to a wild-type reference sequence (HIV-1 HXB2). Based on previous studies from our setting ^[12,24], it was assumed that subjects did not harbor baseline resistance prior to HAART initiation.

Statistical analyses

The primary outcome measure was time to the detection of antiretroviral resistance, defined as the time from the HAART initiation date to the collection date of the first plasma sample containing at least one resistance mutation. Since patients could initiate HAART with a number of different HAART combinations and could switch antiretroviral drugs in their HAART combination frequently throughout follow-up, we conducted stratified analyses where we examined the time to the development of resistance to each major resistance category. Specifically, HIV isolates were assigned to one of four resistance categories based on a modification of the IAS-USA table: any PI resistance (30N, 46I/L, 48V, 50L/V, 54V/L/M, 82A/F/S/T, 84V, or 90M); any NNRTI resistance (100I, 103N, 106A/M, 108I, 181C/I, 188C/H/L, 190A/S, P225H, M230L or 236L); lamivudine (3TC) resistance (184I/V); and any other NRTI resistance (41L, 62V, 65R, 67N, 69D or insertion, 70R, 74V, 75I, 151M, 210W, 215F/Y or 219E/Q) ^[25]. Consistent with previous analyses ^[12,24], 3TC resistance was classified in a separate category due to the common appearance of the M184V/I and the lack of NRTI cross-resistance conferred by this mutation.

Kaplan-Meier analyses

As a preliminary analysis, we were interested to know if there were differences between IDUs and non-IDUs in the rate of resistance. Here, we used Kaplan-Meier analyses which stratified patients by history of injection drug use. We then examined the cumulative rate of the emergence of antiretroviral resistance to each of the four resistance categories described. Survival curves were compared using the log-rank test.

Cox regression analyses

We were aware that IDUs and non-IDUs may have been prescribed different HAART regimens, may discontinue therapy at different rates, or may have different adherence levels ^[4,23], and we therefore sought to confirm that potential underlying differences between IDUs and non-IDUs were not masked by these potential confounders. We therefore used multivariate analyses to examine whether adjustment for clinical and behavioural characteristics helped to uncover potential differences in the rate of resistance between IDUs and non-IDUs. Here, Cox proportional hazard regression models were prepared to examine unadjusted and adjusted relative hazards of antiretroviral resistance among patients with and without a history of injection drug use. As above, separate models were prepared to examine whether having history of injection drug use was associated with differing rates of resistance to PIs, NNRTIs, 3TC, or non-3TC NRTIs.

Adjusted Cox models were prepared using an a priori defined protocol of including history of injection drug use and adjusting for baseline plasma HIV RNA (per log₁₀), CD4 count (per 100 × 10⁶ cells/l), age (per 10 years), gender (male versus female), prescription refill adherence during the first year of HAART (≥ 95% versus < 95%) ^[4,21], and if the relevant antiretroviral drug (PI, NNRTI, 3TC, or non-3TC NRTI) was present in the initial HAART regimen. Since it has been shown that patients commonly change their antiretroviral combination ^[26] or may take breaks from therapy, we also adjusted for the number of months each patient was on the relevant drug class (PI, NNRTI, 3TC, or non-3TC) during follow-up. All tests of significance were two-sided, with a P-value < 0.05 indicating statistical significance. Event-free subjects were censored at the date of their last plasma HIV RNA measurement in the 30 months after the initiation of HAART or the last measure prior to 31 March 2002 if participant's follow-up was less than 30 months prior to this time.

Results

Between 1 August 1996 and 30 September 1999, 1312 antiretroviral-naive participants aged 18 years and older began triple combination therapy containing either a PI or an NNRTI. Of these, 121 (9.2%) were excluded for not having pre-therapy CD4 and HIV RNA data available. The study sample was therefore made up of the remaining 1191 subjects [1004 (84.3%) men, 187 (15.7%) women]. As shown in Table 1, IDUs (n = 335) were more likely to be female (23 versus 13%; P < 0.001), AIDS free at baseline (91 versus 85%; P = 0.009), and to have higher CD4 cell counts (290 versus 270 × 10⁶ cells/l; P = 0.009) in comparison witho non-IDUs (n = 856). Study subjects received a total of 26 different initial therapy combinations, corresponding to one of the following four classifications: 3TC + other NRTI + PI (n = 839; 70.4%), 3TC + other nRTI + NNRTI (n = 214; 18.0%), two NRTI + NNRTI (n = 92; 7.7%), two NRTI + PI (n = 46; 3.9%).

Table 1

Detection of antiretroviral resistance

There were a median two genotypes per study subject (range, 0-13). A total of 360 individuals (32.2%) maintained plasma viral load < 1000 copies/ml over the entire study follow-up and were censored as non-events at the last HIV RNA measure < 1000 copies/ml. Resistance mutations were observed in 298 (25%) subjects. Overall, 3TC resistance was most commonly observed (n = 204; 68.5%), followed by NNRTI (n = 120; 40.3%), NRTI (n = 98; 32.9%) and PI (n = 68; 22.8%) resistance (note that due to multi-category resistance these totals exceed 100%). Among the 298 subjects who developed at least one resistance mutation, the median time to resistance was 8.2 months.

Kaplan-Meier analyses

As shown in Figure 1, the cumulative rate of first PI resistance (a) was 5.4% among non-IDUs and was 5.6% among IDUs at 24 months after the initiation of HAART (log-rank P = 0.983). Conversely, the cumulative rate of first NNRTI resistance (b) was 8.8% among non-IDUs and was 12.6% among IDUs at 24 months after the initiation of HAART (log-rank P = 0.013). As shown in Figure 2, the cumulative rate of 3TC resistance (a) was 17.2% among non-IDUs and was 20.3% among IDUs at 24 months after the initiation of HAART (log-rank P = 0.148). Similarly, the cumulative rate of non-3TC NRTI resistance (b) was 8.9% among non-IDUs and was 6.7% among IDUs at 24 months after the initiation of HAART (log-rank P = 0.387).

Fig. 1

Fig. 2

Cox regression analyses

As shown in Table 2, in separate unadjusted Cox regression models, patients with a history of injection drug use had an unadjusted relative hazard (RH) of PI resistance of 1.0 [95% confidence interval (CI), 0.6-1.7; P = 0.983], an RH of NNRTI resistance of 1.6 (95% CI, 1.1-2.3; P = 0.014), an RH of 3TC resistance of 1.2 (95% CI,: 0.9-1.7; P = 0.149), and an RH of non-3TC nucleotide resistance of 0.8 (95% CI, 0.5-1.3; P = 0.388).

Table 2

In the adjusted model that considered time to PI resistance, after adjustment for PI use in the initial HAART regimen, age, gender, plasma HIV-1 RNA (per log increase), baseline CD4 cell count (per 100 cell increase), months on a PI-containing regimen during follow-up, and adherence (≥ 95% versus < 95%), history of injection drug use was not statistically associated with the time to PI resistance (RH = 0.9; 95% CI, 0.5-1.6; P = 0.766). In a separate adjusted Cox model that considered the time to NNRTI resistance, after adjustment for NNRTI use in the initial HAART regimen, age, gender, plasma HIV-1 RNA (per log increase), baseline CD4 cell count (per 100 cell increase), months on a NNRTI-containing regimen during follow-up, and adherence (≥ 95% versus < 95%), history of injection drug use was not independently associated with the time to NNRTI resistance (RH = 1.5; 95% CI, 1.0-2.2; P = 0.050).

In the model that considered the time to first 3TC resistance, after adjustment for 3TC use in the initial HAART regimen, age, gender, plasma HIV-1 RNA (per log increase), baseline CD4 cell count (per 100 cell increase), months on a PI-containing regimen during follow-up, and adherence (≥ 95% versus < 95%), history of injection drug use was not statistically associated with the time to 3TC resistance (RH = 1.1; 95% CI, 0.8-1.6; P = 0.377). In a separate Cox model that considered the time to first non-3TC nRTI resistance, after adjustment for non-3TC nRTI use in the initial HAART regimen, age, gender, plasma HIV-1 RNA (per log increase), baseline CD4 cell count (per 100 cell increase), months on a non-3TC NRTI-containing regimen during follow-up, and adherence (≥ 95% versus < 95%), history of injection drug use was not statistically associated with the time to non-3TC NRTI resistance (RH = 0.7; 95% CI, 0.4-1.1; P = 0.095).

Discussion

The present analyses demonstrate that among an unselected population-based cohort of antiretroviral-naive patients initiating HAART, history of injection drug use was not independently associated with the time to antiretroviral resistance when patients with and without a history of injection drug use were compared.

When crude data were examined, we did find that the rate of NNRTI resistance was significantly higher among IDUs, and that a marginal difference persisted after statistical adjustment (HR = 1.5; P = 0.05). Although this demonstrates a slightly elevated rate of NNRTI resistance among IDUs, it is important to put this difference into context since the crude data showed that the actual rate of NNRTI resistance was 8.8% among non-IDUs and was 12.6% among IDUs after 24 months of HAART. When interpreting this 4% difference it is important to acknowledge that a difference exists, but that this may be a case of statistical significance rather than clinical significance, particularly because adjusting for relevant potential confounders appeared to diminish this difference rather than unmasking clinically relevant differences.

Our findings are highly relevant to the previous studies that have indicated that physicians may be reluctant to prescribe antiretrovirals to HIV-infected IDUs due to the common belief that IDUs may have lower levels of adherence, which in turn may lead to elevated rates of antiretroviral resistance ^[9,10,13-15]. This concern may partially explain why patients with a history of injection drug use have lower rates of access to HAART, and why many HIV-infected patients die without ever accessing HAART, even in settings with free HIV/AIDS care ^[18,22]. However, previous studies have shown that physicians may be inadequate judges of patient adherence ^[15]. More importantly, although studies have indicated that IDUs may have lower rates of adherence to HAART ^[4,27,28], the present study demonstrates that rates of antiretroviral resistance between IDUs and non-IDUs are statistically similar up to 30 months after the initiation of HAART. This finding is probably explained by the fact that many IDUs are sufficiently adherent to select for resistant strains of HIV ^[29]. These findings should help to allay fears among physicians that prescribing HAART to IDUs may result in elevated rates of antiretroviral resistance ^[20].

In the absence of available data, the provision of HAART to IDUs has remained controversial ^[20]. However, the present study indicates that rates of resistance are not elevated when all patients with a history of injection drug use were examined in a province-wide treatment program. These findings are of international significance, given that several international initiatives such as the WHO's '3 × 5 Initiative' are planning to develop strategies to deliver antiretrovirals in a number of areas where HIV is endemic among IDUs ^[30].

It is important to stress that these data arose in a setting where all HIV/AIDS care, antiretrovirals, and laboratory monitoring are available free of charge, and where previous studies have shown that virtually all patients acquire antiretrovirals through a province-wide single centralized source ^[31]. Although there are major socio-economic differences between IDUs and non-IDUs in our setting ^[16,32], our findings may not be generalizable to settings where IDUs have even greater barriers to accessing healthcare ^[16,32]. We should also note that all patients were antiretroviral naive prior to initiating HAART. As such, the present study may be less affected by selection factors that may have compromised the interpretation of data from clinic-based populations or patients treated with non-HAART regimens. Although the observational nature of the present study may be viewed as a limitation, this could also be viewed as an advantage in the present study since we were interested to examine the rates of antiretroviral resistance among patients with and without a history of injection drug use, among a representative community-based population.

The present study represents the first large population-based cohort study to systematically examine if antiretroviral-naive IDUs initiating HAART have higher rates of antiretroviral resistance in comparison with patients who were infected with HIV through other modes. Our findings indicate that rates of resistance to all major classes of antiretrovirals between IDUs and non-IDUs are very similar during the first 30 months of therapy. These findings should help to allay fears among physicians that prescribing HAART to IDUs may result in elevated rates of antiretroviral resistance, and should demonstrate that withholding HAART from IDUs, as a strategy to prevent elevated rates of resistance, is largely unsupported by quantitative evidence.

Acknowledgements

We thank Bonnie Devlin, Elizabeth Ferris, Nada Gataric, Kelly Hsu Myrna Reginaldo, Jennifer Adachi, and Peter Vann for their research and administrative assistance.

Sponsorship: This work was supported by the Michael Smith Foundation for Health Research through a Senior Scholar award to R.H.