Seattle Treatment Education Project: STEP Perspective - Volume 4, Number 3 - October 1992
Winston Cavert, MD

During the past three years we have learned a good deal about how HIV acquires resistance to AZT, about when resistant strains are most likely to arise, and about how to detect AZT resistance in a dependable manner. However, much remains to be learned regarding the significance of AZT resistance for the health of persons infected with HIV. Moreover, we know very little about how AZT-resistant HIV should be treated.

The Biology of AZT Resistance

AZT, ddI, and other dideoxynucleoside drugs inhibit the activity of an essential Human Immunodeficiency Virus enzyme, reverse transcriptase. Using reverse transcriptase, HIV copies its genetic material into a form that can be easily inserted into a chromosome of the human host cell, thus enabling HIV to use the host cell's machinery to replicate. AZT interferes with reverse transcriptase's gene-copying capacity. Resistance develops when HIV evades AZT's interference by reshaping its reverse transcriptase protein through mutations. Each mutation changes a key amino acid of the reverse transcriptase protein, which in turn alters the enzyme's shape, subsequently diminishing AZT's effectiveness.

Initially, four reverse transcriptase mutations contributing to AZT resistance were discovered.[4] Each mutation independently confers a small decrease in AZT sensitivity. As HIV accumulates two or more mutations, its susceptibility to AZT falls markedly. HIV strains possessing all four major mutations demonstrate high-level resistance to AZT in laboratory tissue cultures. In July, at the eighth International Conference on AIDS in Amsterdam, researchers described a fifth major reverse transcriptase mutation contributing to AZT resistance.[5] Additional amino acid changes also were reported, but they probably will prove to be of minor importance.[6,7] Among the five major mutations, a couple tend to precede the others that observation may eventually provide us with an early warning system to detect small changes in AZT susceptibility before high-level resistance develops.[8,9]

When AZT Resistance Occurs

Several published studies indicate that AZT-resistant HIV strains are most likely isolated from persons with more advanced HIV illness, with lower absolute CD4 cell counts, or with a longer time on AZT treatment.[3,10,11,12] High-level resistance to AZT has been detected as early as ten months after starting AZT therapy. On the other hand, a number of individuals do not seem to carry AZT-resistant HIV even after taking the drug for several years.[13,14] During the Amsterdam conference, Douglas Mayers tabulated the results of susceptibility studies performed on HIV isolates from 60 individuals receiving AZT. After one year of AZT therapy only about one-fifth of the isolates manifested high-level AZT resistance. After two or more years, about half of the isolates showed such resistance.[13]

Measuring AZT Resistance

Over the past couple of years, two basic methods of measuring resistance to AZT have been established. The first mixes virus isolated from HIV-seropositive individuals with lymphocytes from non-infected persons in wells of a tissue culture plate. AZT is added to the wells at varying concentrations. The relative capacity of the virus to grow in wells with different drug concentrations is measured. This method forms the basis of a standardized assay outlined during the July Amsterdam conference which has been adopted by a half-dozen research laboratories in the United States (including the University of Washington's clinical HIV lab).[15] Although simple in concept, this assay unfortunately is time-consuming and tricky in practice. So pending future improvements, the assay is presently used almost exclusively for investigative clinical research studies, and your doctor can not order this test.

The second method of detecting AZT resistance employs the polymerase chain reaction or DNA sequencing techniques to determine which resistance-conferring reverse transcriptase mutations exist in a viral isolate.[8,16,17] This approach holds great future promise, but is not yet sufficiently standardized to allow use in either large research studies or routine clinical practice.

AZT Resistance and Disease Progression

Two recent preliminary studies suggest a correlation may exist between HIV disease progression and AZT resistance. St. Clair and collaborators identified 12 persons who developed AIDS while enrolled in a multi- center trial of AZT therapy. Those 12 were retrospectively matched with 12 others in the same study who had not developed AIDS and who had a similar initial CD4 count. Four of those who developed AIDS harbored AZT-resistant HIV strains (although the other eight did not). All HIV isolated from the control group was sensitive to the drug.[18] Spector reports somewhat analogous results in a handful of children infected with HIV.[19] It should be noted, however, that both of these studies lack enough participants to have statistical power, and both studies suffer from multiple methodological difficulties. Nevertheless, even if these studies proved an ironclad association between AZT resistance and worsening illness, the question of which causes which would remain unanswered.[20] In essence, the question is a classic "the chicken or the egg" riddle: Which comes first? Does acquiring AZT resistance perhaps lead to deteriorating health by permitting HIV to escape the inhibitory effect of the drug? Or alternatively, does advancing illness--accompanied by greater amounts of HIV in the body--enable resistance to develop by increasing the chance that reverse transcriptase mutations occur? Solving this puzzle will prove important not only for assessing the significance of AZT resistance in persons taking that drug, but may also guide our appraisals of the significance of resistance to other antiretrovirals.

AZT Resistance and HIV Virulence

Recently a few investigators suggested AZT resistance may not be of key importance in and of itself. Instead, Boucher and colleagues propose that detectable AZT resistance may instead be a secondary marker for HIV strains that are more aggressive and virulent in other ways.[21] Although other HIV researchers express skepticism, this hypothesis also merits speedy investigation.

Treatment Questions

Since we presently lack understanding regarding the clinical significance of AZT resistance, it follows that we also lack knowledge of when, if, or how to alter anti-HIV therapy in the presence of suspected or documented resistance. Without such guidelines, most physicians and other health care providers now make therapy recommendations based on an intuitive leap from modern medicine's well-traveled experience with bacterial infections. They reason that if they diagnose, for example, a bacterial pneumonia caused by a bacteria resistant to an antibiotic, the antibiotic choice should be changed, or the patient will do poorly. Similarly, we say, if I suspect an individual carries AZT-resistant HIV, I should recommend changing antiretroviral therapy. As an intuitive approach, without scientific studies, this perhaps makes some sense.

But viruses--especially HIV--behave differently than bacteria. In addition to the issues discussed above, HIV exists in most persons as a mix of strains, sometimes termed "quasi-species."[20] Many people who take AZT for the long term probably host a combination of both AZT- sensitive and AZT-resistant quasi-species. Moreover, one laboratory experiment documented that even an HIV isolate well-known to be "AZT- resistant" showed more inhibition in tissue culture containing ddI plus AZT, as compared to ddI alone.[22]

It is true that recent preliminary analyses of two studies, one American and one Italian, suggest that individuals on long-term AZT therapy may gain some advantage by switching to ddI.[23,24] But the relationship between these possible health benefits and AZT susceptibility has not yet been addressed. Moreover, these two studies do not illuminate the issues of when to change therapy or whether or not to continue AZT as well. If, as many expect, AZT resistance proves important, then AZT resistance tests may eventually help health care providers individualize therapy recommendations to persons infected with HIV.

Studying the Clinical Significance of AZT Resistance

During the past year, the issue of HIV drug resistance emerged as a high priority for AIDS research. Several large combination therapy trials presently under way in the United States and elsewhere are designed to help answer some of the questions surrounding AZT resistance, along with these studies' other goals. But many researchers agree that the fundamental questions about AZT resistance and may best be addressed through a prospective clinical trial.

The University of Washington AIDS Clinical Trials Unit (ACTU) leads a national, multi-center study investigating AZT resistance. Sponsored by the National Institute of Allergy and Infectious Diseases, the study (ACTG protocol #194) focuses on two central questions: 1) What is the clinical significance of AZT resistance for the individual on long-term AZT therapy? 2) If AZT resistance is important, how should it be treated?

The study targets people with HIV infection who have taken AZT for at least one year and have not received ddI or ddC. After screening and enrollment, eligible participants will be assigned to one of three double- blinded treatment regimens: Continue AZT alone, switch to ddI alone, or receive both AZT and ddI. The study lasts four months. AZT susceptibility will be determined for every participant. At the end of the study, participants may learn their AZT susceptibility status. All tests and study drugs are free of charge.

If you wish further information regarding this new study, contact the ACTU at (206) 223-3184.

References:

1. Larder BA, et al. Science 1989; 243:1731.

2. Rooke R, et al. AIDS 1989; 3:411.

3. Land S, et al. J Infect Dis 1990; 161:326.

4. Larder BA, et al. Science 1989; 246:1155.

5. Kellam P, et al. Internl Conf on AIDS VIII, July 1992, Abstr ThA1568.

6. Levantis P, et al. Ibid. Abstr ThA 1570.

7. Wainberg, MA, et al. Ibid. Abstr PoA 2403.

8. Tijnagel J, et al. Ibid. Abstr ThA 1571.

9. Boucher CAB, et al. J Infect Dis 1992; 165:105.

10. Richman DD, et al. J AIDS 1990; 3:743.

11. Japour AJ, et al. Internl Conf on AIDS VIII, July 1992, Abstr PoB 3572.

12. Maxeiner, HG, et al. Ibid. Abstr PuB 7345.

13. Mayers, D, et al. Ibid. Abstr PoB 3574.

14. Tudor-Williams B, et al. Lancet 1992; 339:15.

15. Japour AJ, et a. Internl Conf on AIDS VIII, July 1992, Abstr PoA 2484.

16. Kaye S, et al. Ibid. Abstr PoB 3327.

17. Richman DD, et al. J Infect Dis 1991; 164:1075.

18. St. Clair MH, et al. Internl Conf on AIDS VIII, July 1992, Abstr PoB 3578.

19. Spector SA, et al. NIH Second Wrkshp on Viral Resistance, May 1992, Abstr.

20. Richman DD. Rev Infect Dis 1990; 12(S5):S507.

21. Boucher CAB, et al. Internl Conf on AIDS VIII, July 1992, Abstr PoB 3570.

22. Johnson VA, et al. J Infect Dis 1991; 164:646.

23. Kahn J, et al. Internl Conf on AIDS VIII, July 1992, Abstr MoB 0079.

24. Floridia M, et al. Ibid. Abstr MoB 0082.

Copyright (c) 1992 - Seattle Treatment Education Project. Non- Commercial Reproduction Encouraged.
9210
STEP4303

Copyright © 1992 - Seattle Treatment Education Project (STEP) - All rights reserved. Noncommercial reproduction is encouraged. STEP is published four times a year by the Seattle Treatment Education Project, 127 Broadway East, 3rd Floor, Seattle, WA 98102.    Email: step100@aol.com  STEP web page