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Susceptibility to nucleoside analogues of zidovudine-resistant isolates of human immunodeficiency virus
Susceptibility to Nucleoside Analogues of Zidovudine-Resistant Isolates of Human Immunodeficiency Virus DOUGLAS
D. RICHMAN, M.D. San
Diego, Ca/iforn/a
The emergence of human immunodeficiency virus resistant to 3’-azido-3’-deoxythymidine (zidovudine, AZT) in patients with acquired immunodeficiency syndrome (AIDS) or AIDS-related complex has been documented. Isolates from non-AZT-treated persons or those who had received AZT for less than six months showed a narrow range of susceptibility to the drug; on the other hand, isolates from those who had received AZT for six months or more consistently showed reduced susceptibility. Five highly AZT-resistant isolates were also insensitive to other compounds containing a 3’-azido group. No cross-resistance was found to other nucleoside analogues, including 2’,3’-dideoxycytidine and 2’,3’-dideoxyinosine. That cross-resistance occurred only in compounds containing a 3’-azido group suggests that mutations in the reverse transcriptase gene prohibit the enzyme from using nucleoside triphosphate containing a 3’-azido group. Progressive, stepwise increases in resistance have been associated with the sequential accumulation of specific amino acid changes in the reverse transcriptase gene. It is not yet known whether the resistant phenotype as determined in dtro results in clinical resistance to AZT. The gradual appearance of resistant isolates, the variable course of human immunodeficiency virus infections, and the absence of a consistent pattern of resurgent p24 antigen will make the emergence of AZT resistance difficult to correlate with clinical status or other markers. The combination of AZT with other drugs that do not share cross-resistance is a promising area for investigation to identify regimens that are more active and less likely to induce resistance.
I
ecent research has documented the emergence of R human immunodeficiency virus (HIV) resistant to 3’-azidoS’-deoxythymidine (zidovudine, AZT) during
I From the Departments of Pathology and Medicine, University of California, San Diego, and San Diego Veterans Administration Medical Center, San Dlego, California. Requests for reprints should be addressed to Douglas D. Richman, M.D., Infectious Diseases, 1 IIF, San Diego Veterans Administration Medical Center, 3350 La Jolla Village Drive, San Diego, California 92161.
50-8s
May 21, 1990
The American Journal of Medicine
prolonged administration of the drug [l]. This finding was predicted as theoretically likely, because the emergence of resistant virus is increased as the number of replicating units increases, by the chronicity of infection, and by underlying immunodeficiency. Each of these conditions exists in advanced HIV infection. In addition, antiviral drug resistance had been documented for several other viruses, including several herpesviruses to nucleosides and influenza A virus to amantadine/rimantadine [2-71. These observations prompted the search for resistance, as did the clinical observation that the activity of AZT may diminish after the first year of therapy [S] and that HIV is incompletely suppressed during AZT therapy [g-12]. In the initial study, 46 high-titer virus stocks were successfully prepared from 33 persons at various stages of infection with HIV [l]. Of the 46 isolates, 18 were from persons who had never received AZT. The remainder were from patients who had received AZT for acquired immunodeficiency syndrome (AIDS) or AIDS-related complex for varying periods of time ranging from less than six months to more than 18 months. Testing for sensitivity to AZT and other inhibitors could not be accomplished reliably with conventional assays [13]. This problem was solved by the availability of an assay based on syncytial plaque reduction on CD4+ HeLa cell monolayers [14]. This assay was used successfully to evaluate the susceptibility of isolates to AZT and several other HIV inhibitors [1,131. Isolates from persons who had never received AZT displayed a narrow range of susceptibility to the drug (Table I) [l]. Fifty percent inhibitory dose (ID& values ranged from 0.01 to 0.05 PM with mean and median values of 0.03 PM. This narrow range was found in untreated subjects at ‘all stages of HIV infection, from those patients who were asymptomatic to those with late-stage AIDS. Furthermore, this range served as the baseline for susceptibility studies of isolates from AZT-treated patients. Isolates from persons who had received AZT for less than six months were not significantly different in susceptibility from isolates obtained from untreated patients. Isolates from 15 patients who had received AZT therapy for at least six months, however, showed some reduction in susceptibility to AZT. In some cases the changes in IDS0 values were small and were in the range found for pretherapy isolates. In other cases, i.e., isolates from one third of the subjects, they exceeded 100fold increases in ID,, values after exposure to AZT. In addition, isolates taken from the same patient at different stages of AZT therapy displayed pro-
Volume 88 (suppl 5B)
SYMPOSIUM
gressive, stepwise increases in resistance (Figure 1). The discovery of AZT-resistant isolates raised the question of whether the development of in vitro resistance to AZT would extend to other antiretroviral compounds. The answer to this question was determined by testing the susceptibility of paired isolates from persons showing the most significant increases in resistance to AZT (greater than loo-fold increase in ID5,, value) to several HIV inhibitors [13]. Matched isolates (i.e., one obtained early in therapy and sensitive to AZT, the other obtained late in therapy and resistant to AZT) were tested with the CD4+ HeLa cell plaque reduction assay (Table II) [1,131. These data reveal an extremely narrow range of cross-resistance. Late therapy isolates resistant to AZT were also resistant to 3’-azido-2’,3’-dideoxyuridine and 3’-azido-2’,3’-dideoxyguanosine, both of which contain a 3’-azido group. By contrast, the resistant strains were susceptible to other nucleoside analogues including 2’,3’-dideoxycytidine (ddC), and 2’?3’-dideoxy-2’,3’-dide2’,3’-dideoxyinosine, hydrocytidine; to the thymldme analogues, 2’,3’dideoxy-2’,3’-didehydrothymidine and 3’-fluoro-3’deoxythymidine; and to phosphonoformate. The high level resistance of late-therapy isolates to AZT appears to be the end result of the sequential accumulation of multiple amino acid mutations in the reverse transcriptase gene [El. This would explain the progressive, stepwise increases in resistance observed over time. The interesting finding that crossresistance occurred only in compounds containing a 3’azido group (AZT, 3’-azido-2’,3’-dideoxyuridine, 3’azido-2’,3’-dideoxyguanosine) and not in a number of thymidine analogues suggests that mutations in the reverse transcriptase gene no longer permitted the enzyme to use nucleoside triphosphate containing a 3’azido group. Although the exact mechanism by which the mutated reverse transcriptase mediates AZT resistance remains to be determined, it may be possible to treat AZT-resistant strains with other compounds, including nucleoside analogues that do not contain a 3’-azido group. The discovery of AZT-resistant isolates in vitro raises several questions [16]. The most important is whether the resistant phenotype, as determined in vitro, will result in in viva resistance to AZT therapy. The gradual emergence of resistant isolates, coupled with the chronic, progressive, and highly variable course of HIV infections, indicates that their appear-
ON ddC I RICHMAN
TABLE I Correlation of Susceptibility of HIV Isolates to AZT with Duration of Therapy Duration of Therapy (months)
Number of Isolates
None
18
IDso G*M) Mean
Median
Range
0.03 0.03 i
0.03 0.03 0.6 0.07 2
0.007-0.05 0.06-4 0.04-6 0.1-6
l-5 6-11 12-17
i 8 3*
18t
3
0.01-0.05
*Two isolates obtained two months apart from one of the patients in the 181 group showed the same susceptibility (ID,, values of 0.1 PM). Only one value was included. Reprinted with permisslon from [l].
Months
of therapy
igure 1. Comparison of AZT susceptibilitres of HIV isolates obtained before ar after inrtiation of treatment. The upper and lower parts of the figure depict matched IDso and IDg5 values for each isolate. Arrows indicate those isolates obtained before (Pre) or after (Post) initiation of Azi therapy. The maximal IDS, and ID,, values observed for isolates from untreated persons (0.05 yM and 1 yM, respectively) are indicated. Reprinted with permisston from [l].
TABLE II Susceptibility of Paired HIV Isolates to Antiviral Agents Assessed by Syncytial Plaque Reduction Assay in HT4-6C Cells
Isolate
I
A0126 A012D A018A AO18C A0368 A036D P022A PO22C P026A P026B
Duration of AZT Therapy (months)
lE, 1;
AZT
AZdU
A2G
ddC
ddl
0.01
0.4
ND
0.06
ND
i.01
891.0
10”
0.3 0.2
N”:
i.007
500.6
ii
0.6 0.2
5.6 0.03 5.6
560.4 40 0.4
i”5 ;D
0.4 0.5 0.5 0.4
ND
0.1
0.01 2.8
100
AZdU = 3’.azido-2’,3’-dideoxyuridlne; AZG = 3’.azldo-2’,3’-dideoxyguanoslne; ddl = 2’JWdeoxyinosine; hydrothymidine; FddT = 3’.fluoro3’-deoxythymidine; PFA = phosphonoformate; ND = not determined. Adapted with permission from [l]. May 21, 1990
d4C
d4T
FddT
2
ND
1:
22.5
N”D”
:i20
ND
Fi
1.6
ii;
ii
N”D” 0.7 0.7 ND ND
N”D” 0.2 0.2 11
N”: 0.5 0.5 g.5
oY2 0.03 FE
32 28
:; 10
d4C = 2’,3’-dideoxy-2’3’.didehydrocytidlne;
The American Journal of Medicine
PFA
d4T = 2’,3’-dideoxy-2’,3’-dide-
Volume 88 (suppl 55)
5B-9s
SYMPOSIUM
ON ddC I RICHMAN
ante will not be marked by dramatic changes in the clinical status of patients, and in fact may occur undetected. Moreover, the use of p24 antigen levels as a marker of virus replication may be less useful than originally anticipated [9,17]. Patients in whom highly resistant variants were found showed no consistent pattern of resurgent p24 antigen [l]. Another question is whether HIV variants will exhibit reduced virulence and pathogenic potential as does, in general, acyclovir-resistant herpes simplex virus [18-201. The answer to this question is as yet unknown. Other questions to be answered by future research include the following: Does any particular pattern of therapy lead to the selection of more or less resistant strains? Will resistant virus be transmitted? Will sensitive virus re-emerge if AZT therapy is discontinued? What is the time course and sequence of appearance of resistant variants in AZT-treated patients? Is the rate of emergence of resistance in different populations low (e.g., in asymptomatic adults) or high (e.g., in children)? With respect to the last question, preliminary studies show that in patients with CD4’ cell counts greater than.200/mm3, the’ resistant phenotype does develop on prolonged AZT therapy. However, the rate of appearance and degree of resistance are lower in magnitude than those observed in patients with more advanced diseases. The clinical implications of AZT resistance in patients receiving prolonged AZT therapy have not yet been established. Therefore, it would be premature to alter any treatment protocols for HIV-infected patients. It appears that AZT, at least in the short run, will probably not be reelaced, but will be supplemented by nucleosides and other compounds that do not show cross-resistance to AZT. Among the first approaches to the use of a second drug are studies with both AZT and ddC. ddC was the second compound, after AZT, to be shown to inhibit the replication of HIV in humans, as determined by the reduction of p24 antigenemia [21,221. Moreover, the toxicities of AZT and ddC are non-overlappitig. The use of alternating AZT and ddC has been investigated to ascertain if such a regimen could reduce the toxicities of each drug, while maintaining antiviral activity [21,23,24]. In addition, a phase I/II study of combinations of AZT and ddC has been initiated to determine whether the doses and toxicity of either or both drugs could be reduced while maintaining antiviral activity [25]. Whether the alternating or the combination regimens of AZT and ddC can delay or prevent the emergence of drug-resistant virus will prove to be of great interest. It will be increasingly necessary to evaluate new candidate antiretroviral agents for their ability to function in combination with other drugs with regard to activity, toxicity, and resistance. Once promising combinations are identified, the clinical evaluation of multiple drug regimens will be complex and difficult.
56-10s
May 21, 1990
The American Journal of Medicine
Such efforts will be necessary, however, because the future of drug therapy for HIV infection appears to be linked to the development of combination, rather than single, drug regimens. REFERENCES 1. Larder EA. Darbv G. Richman DD: HIV with reduced sensitivitv to zldovudine isolated during prolonged therapy. Science 1989; 243: 1731-1734. ’ 2. Larder BA, Darby G: Virus drug-resistance: mechanisms and consequences. Antiviral Res 1984; 4: 1-42. 3. Barry DW, Nusinoff.Lehrman S, Ellis NM, Diron KK, Furman PA: Viral resistance: clinical experience. Stand J Infect Dis 1985; 47: 155-164. 4. Erice A, Chou S, Biron KK, Stanat SC, Balfour Jr HH, Jordan MC: Progressive disease due to ganciclovir-resistant cytomegalovirus in immunocompromised patients. N Engl J Med 1989; 320: 289-292. 5. Erlich KS, Mills J, Chatis P, et al: Acyclovir-resistant herpes simplex virus infections in patients with the acquired immunodeficiency syndrome. N Engl J Med 1989; 320: 293296. 6. Belshe RB, Burk B, Newman F, Cerruti RL, Sim IS: Resistance of influenza A virus to amantadine and rimantadine: results of one decade of surveillance. J Infect Dis 1989; 159: 430-433. 7. Hayden FG, Belshe RB, Clover RD, Hay Al, Oakes MG, Soo W: Emergence and apparent transmission of rimantadine-resistant influenza A virus in faniilies. N Engl J Med 1989; 321: 1696-1702. 8. [ischl MA, Richman DD, Causey DM, et al: Prolonged zidovudine therapy in patients with AIDS and advanced AIDS-ielated complex. JAMA 1989; 262: 2405-2410. 9. Fischl MA, Richman DD, Grieco MH, et al: The efficacy of azidothymidine (ATT) in the treatment of patients with AIDS and AIDS-related complex: a double-blind, placebocontrolled trial. N Engl J Med 1987; 317: 185-191. 10. Spector SA, Kennedy C, McCutchan JA, et a/: The antiviral effect of zidovudine and ribavirin in clinical trials and the use of ~24 antigen I levels as a virologic marker. J lrifect Dis 1989; 159: 822-828. 11. Ho DD, Moudgil T, Alam M: Quantitation of human immunodeficlency virus type I in the blood of infected persons. N Engi J Med 1989; 321: 1621-1625. 12. Coombs RW, Collier AC, Allain JP, et al: Plasma viremia in human immunodeficiency virus infection. N Engl J Med 1989; 321: 1626-1631. 13. Larder BA, Chesebro B, Richman DD: Susceptibilities of zidovudine-susceptible and -resistant human immunodeficiency virus isolates to antiviral agents determined by using a quantitative plaque reduction assay. Antimicrob Agents Chemother 1990; 34: 436-441. 14. Chesebro B, Wehrly K: Development of a sensitive quantitative focal assay for human immunodeficiency virus infectivity. J Viral 1988; 62: 3779-3788. 15. Larder BA, Kemp SD: Multiple mutations in HIV-I reverse transcriptase confer highlevel resistance to zidovudine (ATT). Science 1989; 246: 1155-1158. 16. Richman DD: Zidovudine resistance of human immunodeflciency virus. Rev Infect Dis 1990; (in press). 17. Jackson GG. Paul DA. Falk LA. et al: Human immunodeficiencv virus fHlV) antiaenemia (~24) in the acqhired immunodefciency syndrome (AIDS) and thkeffectbf treatment with zidovudine (AZT). Ann Intern Med 1988; 108: 175-180. 18. field HJ, Darby G: Pathogenicity in mice of strains of herpes simplex virus which are resistant to acyclo\ilr in vitro and in viva. Antimicrob Agents Chemother 1980; 17: 209216. 19. Larder BA, Darby G: Selection and characterisation of acyclovir-resistant herpes simplex virus type 1 mutants inducing altered DNA polymerase activities. Virology 1985; 146: 262-271. 20. Darby G, Field HJ, Salisbury SA: Altered substrate specificity of herpes simplex virus thymidine kinase confers acyclovir-resistance. Nature 1981; 289: 81-83. 21. Yarchoan R, Thomas RV, Allain JP, et at Phase I studies of 2’,3’-dideoxycytidine in severe human immunodeficiency virus infection as a single agent and alternating with zidovudine (ATT). Lancet 1988; I: 76-80. 22. Merigan TC, Skowron G, Bozzette SA, et a/: Circulating p24 antigen levels and resoonses to dideoxvcvtidine in human immunodeficiencv virus (HIV) Infections: a DhaSe I &d II study. Ann in&n Med 1989; 110: 189-194. . 23. Skowron G, Merigan TC: Alternating and intermittent regimens of zidovudine (3’.azido3’.deoxythymidine) and dideoxycytidine (2’,3’-dideoxycytidine) in the treatment of patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex. Am J Med 1990; 88 (suppl 5B): 5B-20S-5B-23s. 24. Bozzette SA, Richman DD: Salvage therapy for zidovudine-intolerant HIV-infected patients with alternating and intermittent regimens of zidovudine and dideoxycytidine. Am J Med 1990; 88 (suppl 58): 58.24S-58-268. 25. Meng T-C, Fischl MA, Richman DD: AIDS Clinical Trials Group, phase i/II study of combination 2’,3’-dideoxycytidine and zidovudine in patients with acquired immunodeficiency syndrome (AIDS) and advanced AIDS-related complex. Am J Med 1990; 88 (suppl 55): 5B-27S-5B-30s.
Volume 88 (suppl 58)
D. RICHMAN, M.D. San
Diego, Ca/iforn/a
The emergence of human immunodeficiency virus resistant to 3’-azido-3’-deoxythymidine (zidovudine, AZT) in patients with acquired immunodeficiency syndrome (AIDS) or AIDS-related complex has been documented. Isolates from non-AZT-treated persons or those who had received AZT for less than six months showed a narrow range of susceptibility to the drug; on the other hand, isolates from those who had received AZT for six months or more consistently showed reduced susceptibility. Five highly AZT-resistant isolates were also insensitive to other compounds containing a 3’-azido group. No cross-resistance was found to other nucleoside analogues, including 2’,3’-dideoxycytidine and 2’,3’-dideoxyinosine. That cross-resistance occurred only in compounds containing a 3’-azido group suggests that mutations in the reverse transcriptase gene prohibit the enzyme from using nucleoside triphosphate containing a 3’-azido group. Progressive, stepwise increases in resistance have been associated with the sequential accumulation of specific amino acid changes in the reverse transcriptase gene. It is not yet known whether the resistant phenotype as determined in dtro results in clinical resistance to AZT. The gradual appearance of resistant isolates, the variable course of human immunodeficiency virus infections, and the absence of a consistent pattern of resurgent p24 antigen will make the emergence of AZT resistance difficult to correlate with clinical status or other markers. The combination of AZT with other drugs that do not share cross-resistance is a promising area for investigation to identify regimens that are more active and less likely to induce resistance.
I
ecent research has documented the emergence of R human immunodeficiency virus (HIV) resistant to 3’-azidoS’-deoxythymidine (zidovudine, AZT) during
I From the Departments of Pathology and Medicine, University of California, San Diego, and San Diego Veterans Administration Medical Center, San Dlego, California. Requests for reprints should be addressed to Douglas D. Richman, M.D., Infectious Diseases, 1 IIF, San Diego Veterans Administration Medical Center, 3350 La Jolla Village Drive, San Diego, California 92161.
50-8s
May 21, 1990
The American Journal of Medicine
prolonged administration of the drug [l]. This finding was predicted as theoretically likely, because the emergence of resistant virus is increased as the number of replicating units increases, by the chronicity of infection, and by underlying immunodeficiency. Each of these conditions exists in advanced HIV infection. In addition, antiviral drug resistance had been documented for several other viruses, including several herpesviruses to nucleosides and influenza A virus to amantadine/rimantadine [2-71. These observations prompted the search for resistance, as did the clinical observation that the activity of AZT may diminish after the first year of therapy [S] and that HIV is incompletely suppressed during AZT therapy [g-12]. In the initial study, 46 high-titer virus stocks were successfully prepared from 33 persons at various stages of infection with HIV [l]. Of the 46 isolates, 18 were from persons who had never received AZT. The remainder were from patients who had received AZT for acquired immunodeficiency syndrome (AIDS) or AIDS-related complex for varying periods of time ranging from less than six months to more than 18 months. Testing for sensitivity to AZT and other inhibitors could not be accomplished reliably with conventional assays [13]. This problem was solved by the availability of an assay based on syncytial plaque reduction on CD4+ HeLa cell monolayers [14]. This assay was used successfully to evaluate the susceptibility of isolates to AZT and several other HIV inhibitors [1,131. Isolates from persons who had never received AZT displayed a narrow range of susceptibility to the drug (Table I) [l]. Fifty percent inhibitory dose (ID& values ranged from 0.01 to 0.05 PM with mean and median values of 0.03 PM. This narrow range was found in untreated subjects at ‘all stages of HIV infection, from those patients who were asymptomatic to those with late-stage AIDS. Furthermore, this range served as the baseline for susceptibility studies of isolates from AZT-treated patients. Isolates from persons who had received AZT for less than six months were not significantly different in susceptibility from isolates obtained from untreated patients. Isolates from 15 patients who had received AZT therapy for at least six months, however, showed some reduction in susceptibility to AZT. In some cases the changes in IDS0 values were small and were in the range found for pretherapy isolates. In other cases, i.e., isolates from one third of the subjects, they exceeded 100fold increases in ID,, values after exposure to AZT. In addition, isolates taken from the same patient at different stages of AZT therapy displayed pro-
Volume 88 (suppl 5B)
SYMPOSIUM
gressive, stepwise increases in resistance (Figure 1). The discovery of AZT-resistant isolates raised the question of whether the development of in vitro resistance to AZT would extend to other antiretroviral compounds. The answer to this question was determined by testing the susceptibility of paired isolates from persons showing the most significant increases in resistance to AZT (greater than loo-fold increase in ID5,, value) to several HIV inhibitors [13]. Matched isolates (i.e., one obtained early in therapy and sensitive to AZT, the other obtained late in therapy and resistant to AZT) were tested with the CD4+ HeLa cell plaque reduction assay (Table II) [1,131. These data reveal an extremely narrow range of cross-resistance. Late therapy isolates resistant to AZT were also resistant to 3’-azido-2’,3’-dideoxyuridine and 3’-azido-2’,3’-dideoxyguanosine, both of which contain a 3’-azido group. By contrast, the resistant strains were susceptible to other nucleoside analogues including 2’,3’-dideoxycytidine (ddC), and 2’?3’-dideoxy-2’,3’-dide2’,3’-dideoxyinosine, hydrocytidine; to the thymldme analogues, 2’,3’dideoxy-2’,3’-didehydrothymidine and 3’-fluoro-3’deoxythymidine; and to phosphonoformate. The high level resistance of late-therapy isolates to AZT appears to be the end result of the sequential accumulation of multiple amino acid mutations in the reverse transcriptase gene [El. This would explain the progressive, stepwise increases in resistance observed over time. The interesting finding that crossresistance occurred only in compounds containing a 3’azido group (AZT, 3’-azido-2’,3’-dideoxyuridine, 3’azido-2’,3’-dideoxyguanosine) and not in a number of thymidine analogues suggests that mutations in the reverse transcriptase gene no longer permitted the enzyme to use nucleoside triphosphate containing a 3’azido group. Although the exact mechanism by which the mutated reverse transcriptase mediates AZT resistance remains to be determined, it may be possible to treat AZT-resistant strains with other compounds, including nucleoside analogues that do not contain a 3’-azido group. The discovery of AZT-resistant isolates in vitro raises several questions [16]. The most important is whether the resistant phenotype, as determined in vitro, will result in in viva resistance to AZT therapy. The gradual emergence of resistant isolates, coupled with the chronic, progressive, and highly variable course of HIV infections, indicates that their appear-
ON ddC I RICHMAN
TABLE I Correlation of Susceptibility of HIV Isolates to AZT with Duration of Therapy Duration of Therapy (months)
Number of Isolates
None
18
IDso G*M) Mean
Median
Range
0.03 0.03 i
0.03 0.03 0.6 0.07 2
0.007-0.05 0.06-4 0.04-6 0.1-6
l-5 6-11 12-17
i 8 3*
18t
3
0.01-0.05
*Two isolates obtained two months apart from one of the patients in the 181 group showed the same susceptibility (ID,, values of 0.1 PM). Only one value was included. Reprinted with permisslon from [l].
Months
of therapy
igure 1. Comparison of AZT susceptibilitres of HIV isolates obtained before ar after inrtiation of treatment. The upper and lower parts of the figure depict matched IDso and IDg5 values for each isolate. Arrows indicate those isolates obtained before (Pre) or after (Post) initiation of Azi therapy. The maximal IDS, and ID,, values observed for isolates from untreated persons (0.05 yM and 1 yM, respectively) are indicated. Reprinted with permisston from [l].
TABLE II Susceptibility of Paired HIV Isolates to Antiviral Agents Assessed by Syncytial Plaque Reduction Assay in HT4-6C Cells
Isolate
I
A0126 A012D A018A AO18C A0368 A036D P022A PO22C P026A P026B
Duration of AZT Therapy (months)
lE, 1;
AZT
AZdU
A2G
ddC
ddl
0.01
0.4
ND
0.06
ND
i.01
891.0
10”
0.3 0.2
N”:
i.007
500.6
ii
0.6 0.2
5.6 0.03 5.6
560.4 40 0.4
i”5 ;D
0.4 0.5 0.5 0.4
ND
0.1
0.01 2.8
100
AZdU = 3’.azido-2’,3’-dideoxyuridlne; AZG = 3’.azldo-2’,3’-dideoxyguanoslne; ddl = 2’JWdeoxyinosine; hydrothymidine; FddT = 3’.fluoro3’-deoxythymidine; PFA = phosphonoformate; ND = not determined. Adapted with permission from [l]. May 21, 1990
d4C
d4T
FddT
2
ND
1:
22.5
N”D”
:i20
ND
Fi
1.6
ii;
ii
N”D” 0.7 0.7 ND ND
N”D” 0.2 0.2 11
N”: 0.5 0.5 g.5
oY2 0.03 FE
32 28
:; 10
d4C = 2’,3’-dideoxy-2’3’.didehydrocytidlne;
The American Journal of Medicine
PFA
d4T = 2’,3’-dideoxy-2’,3’-dide-
Volume 88 (suppl 55)
5B-9s
SYMPOSIUM
ON ddC I RICHMAN
ante will not be marked by dramatic changes in the clinical status of patients, and in fact may occur undetected. Moreover, the use of p24 antigen levels as a marker of virus replication may be less useful than originally anticipated [9,17]. Patients in whom highly resistant variants were found showed no consistent pattern of resurgent p24 antigen [l]. Another question is whether HIV variants will exhibit reduced virulence and pathogenic potential as does, in general, acyclovir-resistant herpes simplex virus [18-201. The answer to this question is as yet unknown. Other questions to be answered by future research include the following: Does any particular pattern of therapy lead to the selection of more or less resistant strains? Will resistant virus be transmitted? Will sensitive virus re-emerge if AZT therapy is discontinued? What is the time course and sequence of appearance of resistant variants in AZT-treated patients? Is the rate of emergence of resistance in different populations low (e.g., in asymptomatic adults) or high (e.g., in children)? With respect to the last question, preliminary studies show that in patients with CD4’ cell counts greater than.200/mm3, the’ resistant phenotype does develop on prolonged AZT therapy. However, the rate of appearance and degree of resistance are lower in magnitude than those observed in patients with more advanced diseases. The clinical implications of AZT resistance in patients receiving prolonged AZT therapy have not yet been established. Therefore, it would be premature to alter any treatment protocols for HIV-infected patients. It appears that AZT, at least in the short run, will probably not be reelaced, but will be supplemented by nucleosides and other compounds that do not show cross-resistance to AZT. Among the first approaches to the use of a second drug are studies with both AZT and ddC. ddC was the second compound, after AZT, to be shown to inhibit the replication of HIV in humans, as determined by the reduction of p24 antigenemia [21,221. Moreover, the toxicities of AZT and ddC are non-overlappitig. The use of alternating AZT and ddC has been investigated to ascertain if such a regimen could reduce the toxicities of each drug, while maintaining antiviral activity [21,23,24]. In addition, a phase I/II study of combinations of AZT and ddC has been initiated to determine whether the doses and toxicity of either or both drugs could be reduced while maintaining antiviral activity [25]. Whether the alternating or the combination regimens of AZT and ddC can delay or prevent the emergence of drug-resistant virus will prove to be of great interest. It will be increasingly necessary to evaluate new candidate antiretroviral agents for their ability to function in combination with other drugs with regard to activity, toxicity, and resistance. Once promising combinations are identified, the clinical evaluation of multiple drug regimens will be complex and difficult.
56-10s
May 21, 1990
The American Journal of Medicine
Such efforts will be necessary, however, because the future of drug therapy for HIV infection appears to be linked to the development of combination, rather than single, drug regimens. REFERENCES 1. Larder EA. Darbv G. Richman DD: HIV with reduced sensitivitv to zldovudine isolated during prolonged therapy. Science 1989; 243: 1731-1734. ’ 2. Larder BA, Darby G: Virus drug-resistance: mechanisms and consequences. Antiviral Res 1984; 4: 1-42. 3. Barry DW, Nusinoff.Lehrman S, Ellis NM, Diron KK, Furman PA: Viral resistance: clinical experience. Stand J Infect Dis 1985; 47: 155-164. 4. Erice A, Chou S, Biron KK, Stanat SC, Balfour Jr HH, Jordan MC: Progressive disease due to ganciclovir-resistant cytomegalovirus in immunocompromised patients. N Engl J Med 1989; 320: 289-292. 5. Erlich KS, Mills J, Chatis P, et al: Acyclovir-resistant herpes simplex virus infections in patients with the acquired immunodeficiency syndrome. N Engl J Med 1989; 320: 293296. 6. Belshe RB, Burk B, Newman F, Cerruti RL, Sim IS: Resistance of influenza A virus to amantadine and rimantadine: results of one decade of surveillance. J Infect Dis 1989; 159: 430-433. 7. Hayden FG, Belshe RB, Clover RD, Hay Al, Oakes MG, Soo W: Emergence and apparent transmission of rimantadine-resistant influenza A virus in faniilies. N Engl J Med 1989; 321: 1696-1702. 8. [ischl MA, Richman DD, Causey DM, et al: Prolonged zidovudine therapy in patients with AIDS and advanced AIDS-ielated complex. JAMA 1989; 262: 2405-2410. 9. Fischl MA, Richman DD, Grieco MH, et al: The efficacy of azidothymidine (ATT) in the treatment of patients with AIDS and AIDS-related complex: a double-blind, placebocontrolled trial. N Engl J Med 1987; 317: 185-191. 10. Spector SA, Kennedy C, McCutchan JA, et a/: The antiviral effect of zidovudine and ribavirin in clinical trials and the use of ~24 antigen I levels as a virologic marker. J lrifect Dis 1989; 159: 822-828. 11. Ho DD, Moudgil T, Alam M: Quantitation of human immunodeficlency virus type I in the blood of infected persons. N Engi J Med 1989; 321: 1621-1625. 12. Coombs RW, Collier AC, Allain JP, et al: Plasma viremia in human immunodeficiency virus infection. N Engl J Med 1989; 321: 1626-1631. 13. Larder BA, Chesebro B, Richman DD: Susceptibilities of zidovudine-susceptible and -resistant human immunodeficiency virus isolates to antiviral agents determined by using a quantitative plaque reduction assay. Antimicrob Agents Chemother 1990; 34: 436-441. 14. Chesebro B, Wehrly K: Development of a sensitive quantitative focal assay for human immunodeficiency virus infectivity. J Viral 1988; 62: 3779-3788. 15. Larder BA, Kemp SD: Multiple mutations in HIV-I reverse transcriptase confer highlevel resistance to zidovudine (ATT). Science 1989; 246: 1155-1158. 16. Richman DD: Zidovudine resistance of human immunodeflciency virus. Rev Infect Dis 1990; (in press). 17. Jackson GG. Paul DA. Falk LA. et al: Human immunodeficiencv virus fHlV) antiaenemia (~24) in the acqhired immunodefciency syndrome (AIDS) and thkeffectbf treatment with zidovudine (AZT). Ann Intern Med 1988; 108: 175-180. 18. field HJ, Darby G: Pathogenicity in mice of strains of herpes simplex virus which are resistant to acyclo\ilr in vitro and in viva. Antimicrob Agents Chemother 1980; 17: 209216. 19. Larder BA, Darby G: Selection and characterisation of acyclovir-resistant herpes simplex virus type 1 mutants inducing altered DNA polymerase activities. Virology 1985; 146: 262-271. 20. Darby G, Field HJ, Salisbury SA: Altered substrate specificity of herpes simplex virus thymidine kinase confers acyclovir-resistance. Nature 1981; 289: 81-83. 21. Yarchoan R, Thomas RV, Allain JP, et at Phase I studies of 2’,3’-dideoxycytidine in severe human immunodeficiency virus infection as a single agent and alternating with zidovudine (ATT). Lancet 1988; I: 76-80. 22. Merigan TC, Skowron G, Bozzette SA, et a/: Circulating p24 antigen levels and resoonses to dideoxvcvtidine in human immunodeficiencv virus (HIV) Infections: a DhaSe I &d II study. Ann in&n Med 1989; 110: 189-194. . 23. Skowron G, Merigan TC: Alternating and intermittent regimens of zidovudine (3’.azido3’.deoxythymidine) and dideoxycytidine (2’,3’-dideoxycytidine) in the treatment of patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex. Am J Med 1990; 88 (suppl 5B): 5B-20S-5B-23s. 24. Bozzette SA, Richman DD: Salvage therapy for zidovudine-intolerant HIV-infected patients with alternating and intermittent regimens of zidovudine and dideoxycytidine. Am J Med 1990; 88 (suppl 58): 58.24S-58-268. 25. Meng T-C, Fischl MA, Richman DD: AIDS Clinical Trials Group, phase i/II study of combination 2’,3’-dideoxycytidine and zidovudine in patients with acquired immunodeficiency syndrome (AIDS) and advanced AIDS-related complex. Am J Med 1990; 88 (suppl 55): 5B-27S-5B-30s.
Volume 88 (suppl 58)