BHIVA draft adult antiretroviral treatment guidelines: Resistance

INTRODUCTION

Drug resistance testing has been used for many years to support the treatment of infectious diseases. In virology, resistance testing has been constrained, until recently, by a lack of appropriate technology, and difficulty in routinely propagating relevant viruses. Gene sequencing was established as a research tool over 30 years ago and now academic and commercial groups have developed and modified research technologies to form ‘kits’ and/or systems for evaluation for clinical use.

The challenge is to develop a clinical ‘service’ that will detect or predict changes in susceptibility associated with antiretroviral drug resistance, using a system that is rapid, reliable, reproducible and quality assured.

Assuming that technologies available withstand this scrutiny we are left to determine how results of resistance testing are related to virological and clinical outcomes, and their relevance for improving patient care.

Intuitively it would seem obvious that an understanding of HIV-1 drug resistance profiles should help in the management of their therapy. However, within the setting of a chronic viral infection where drug responses are equally influenced by adherence, pharmacological tolerance and drug resistance it may be difficult to demonstrate this unequivocally.

Antiretroviral drug resistance has been described retrospectively, in vitro and in vivo, to be associated with poor virological and clinical outcomes (Table 1a) and short-term prospective, randomized controlled trials have, in some studies, demonstrated the short term benefits of resistance testing over standard of care (Table 1b). It remains to be clarified in which patient groups resistance testing is of long term benefit, or indeed if resistance testing without measures of adherence and/or drug pharmacokinetics, in the long term, is likely to be useful at all.

TABLE 1

1. Retrospective studies

1. F Brun-Vezinet,S Kaye, F.Ferchal, C Loveday,C, Buffet-Janvresse, R Tedder, J Darbyshire, JP Aboulker AZT Resistance:A Case Controlled Study from Concorde. Third Int. Workshop on HIV Drug Resistance.Kauai, Hawaii. 2-5 August 1994. Abstr.49
2. D’Aquila RT,Johnson VA,Welles SL et al.ZDV resistance and HIV-1 disease progression during antiretroviral therapy.Ann Int Med.1995, 122: 401-408
3. Japour AJ, Welles S, D’Aquila RT et al. Prevalence and clinical significance of ZDV resistance mutations in HIV isolates from patients after long term ZDV therapy. J Infect.Dis.1995. 171:1172-1179
4. Miller V, Phillips, Rottman C et al. Dual resistance to ZDV and 3TC in patients treated with ZDV/3TC combination therapy: association with therapy failure. J Inf ect Dis. 1998, 177: 1521-1532
5. Zolopa AR, Shafer R, Warford A et al. HIV-1 genotypic resistance patterns predict response to saquinavir/ritonavir therapy in patients in whom previous protease inhibitor therapy failed. Ann Int Med. 1999, 131: 813-821
6. Deeks SG, Hellman NS, Grant RM et al. Novel 4 drug salvage treatment regimens after failure of HIV-1 protease inhibitor-containing regimen: antiviral activity and correlation of baseline phenotype drug susceptibility to virological outcome. J Infect Dis, 1999, 179: 1375-1381
7. Lorenzi P, Opravil M, Hirschel B et al. Impact of drug resistance mutations on virological response to salvage therapy. AIDS, 1999, 13: F17-21
8. Miller V, Cozzi-Lepri A, Hertogs K et al. HIV drug susceptibility and drug response to Mega-HAART regimen in patients from the Frankfurt cohort. Antiviral Therapy 2000, 5:56-71
9. Van Vaerenbergh K, Van Leuthem K, Van Wijngaerden E et al. Baseline HIV-1 genotypic resistance to a newly added nucleoside analogue is predictive for virological failure of the new therapy. AIDS Res Hum Retroviruses 2000. 16: 529-537

2. Prospective Studies
Resistance Test

MTs = mutations; PI = protease inhibitors; NRTI = nucleoside reverse transcriptase inhibitors; SOC = standard of care; RCT = randomized controlled trial

1. Durant J, Clevenbergh F, Halfon F et al. Drug resistance genotyping in HIV-1 therapy: the VIRADAPT randomised controlled trial. Lancet. 1999, 353: 2195-2199
2. Clevenbergh F, Durant J, Halfon F et al. Persisting long-term benefit of antiretroviral genotypic guided treatment for HIV infe4cted patients failing HAART. Antiviral Ther. 1999 (Suppl 1) 42: Abstr 60
3. Baxter JD, Mayers DL, Wentworth DN et al. A randomised study of antiretroviral management based on plasma genotypic resistance testing in patients failing therapy.CPCRA 046 study team for the Terry Beirn Community Programs for Clinical Research on AIDS. AIDS 2000. 14: F83-93
4. Cohen C, Kessler H, Hunt S et al. Phenotypic resistance testing significantly improves response to therapy: final analysis of a randomized controlled trial (VIRA 3001). Antiviral Ther. 2000. 5 (Suppl 3): 67
5. Havana – CROI, 2001
6. Meynard JL, Wray M, Mourand-Joubert L et al. Impact of treatment guided by phenotypic and genotypic resistance tests on the response to antiretroviral therapy: a randomized controlled trial (NARVAL,ANRS 088). Antiviral Ther. 2000, (Suppl 3): 67-68

THE TESTS

Resistance tests available for determination of of HIV-1 drug susceptibility (phenotyping) and detection of mutations associated with drug resistance (genotyping) are amongst the most complex ever addressed for use in routine clinical care. In terms of reviewing performance and quality these issues divide naturally into:

1. Clinical sample and request. This is an important determinant of the ability to generate a useful result (Table 2)

2. The technical process. A range of different assays are available, summarised in Table 3

3. Validation of the technical process and printout. Assay quality control means the assimilation of extra samples at all stages of the test process, in parallel with clinical samples, in order to identify changes in sensitivity in the assay, and the generation of potentially false results. These extra controls increase the overall cost of the test. External quality assurance schemes test the quality of the overall process, from receipt of sample in the laboratory, to production of the final report. Such schemes are being developed for HIV drug resistance.

All these quality aspects of the test are assessed as part of laboratory accreditation procedures.

4. Generation of clinical report. Genotypic assays produce large volumes of detailed gene sequence information, from which must be identified those changes thought to be important in determining drug susceptibility. This comprises two steps. Firstly, the identification of any mutations which are different from the presumed "drug sensitive" virus, and secondly, providing an interpretation of these changes, with regard to specific drugs.. Software programmes are available to undertake these analyses, however they need regular updating, in the light of new knowledge on drug resistance, and differences between them still exist. A selection of programmes available for providing an interpretation of drug resistance associated mutations is given in Table 4. Table provides guidance on the association of specific mutations with resistance to specific drugs.

5. Clinical interpretation and advice. The factors involved in optimizing the clinical utility of resistance test results are given in Table 6.

Table 2

Practicalities in requesting resistance test

Table 3

(a)

AVAILABLE TESTS

Genotype

Table 3

(b)

Phenotype

Association of specific mutations with drug resistance

Table 5

Most common HIV-1 resistance mutations against currently available drugs

Notes

(1) Time to resistance is very variable and highly dependent on the efficacy of the combination therapy. Therapy choice should not primarily be based on a longer period to resistance since this is often associated with a less powerful drug. Fast: resistance generally develops within weeks of initiation of monotherapy, intermediate: resistance generally develops within a few months of initiation of monotherapy, long: resistance has either been poorly described in the literature or has developed only years after the initiation of monotherapy.

(2) Specific drug related mutations indicated are the ones observed under monotherapy. Mutations shown in bold are generally considered either primary or key mutations, their clinical use in patient follow-up is currently still under investigation. Mutations between brackets are cross-resistance mutations selected during therapy with another drug.

(3) Combination therapy can select for the mutations mentioned as cross-resistance mutations.

(4) There is increasing evidence for at least some degree of cross-resistance between AZT and d4T, as well as other nucleoside analogues.

(5) The phenotypic implications of these mutations have not been added, since assay cut-offs and their biological and clinical implications are still unclear. Clinically relevant cut-offs are only available for lopinavir, for which 10-fold reduced susceptibility is associated with an increased risk of non-response.

Aminoacid codes are: A (alanine), C (cysteine), D (aspartic acid), E (glutamic acid), F (phenylalanine), G (glycine), H (hystidine), I (isoleucine), K (lysine), L (leucine), M (methionine), P (proline), Q (glutamine), R (arginine), S (serine), T (threonine), V (valine), W (tryptophane), Y (tyrosine).

The data in this table are adapted from reference 1. [Vandamme A-M, Houyez F, Banheghi D et al. Laboratory guidelines for the practical use of HIV drug resistance tests in patient follow up. Antiviral Therapy 2001 (in press)].

TABLE 7 RECOMMENDATIONS

Notes: