HIVresistanceWeb - Perspectives and Opinions - Report From the Second International Workshop on Drug Resistance and Treatment Strategies (24-27 June, Lake Maggiore, Italy): Resistance to Reverse Transcriptase Inhibitors

Report From the Second International Workshop on Drug Resistance and Treatment Strategies (24-27 June, Lake Maggiore, Italy): Resistance to Reverse Transcriptase Inhibitors

written by Charles Boucher, M.D.
published on HIVresistanceWeb: July 24, 1998

Nucleoside Analogue Cross-Resistance

Several investigators reported on a novel pathway for the development of multidrug resistance (MDR) to nucleoside analogues. Using different approaches, all investigating groups identified the same novel molecular mechanism which can lead to decreased susceptibility in vitro to AZT, ddI, d4T, 3TC, and adefovir dipivoxil. The key feature of this novel pathway is an insertion of two amino acids at codon 69 of the reverse transcriptase (RT) enzyme. In all reported cases, this insertion was accompanied by well-known AZT mutations (positions 41, 210 and 215); other changes, at codons 62 and 75, were also identified. [1,2]

The finding that this insertion at codon 69 in combination with AZT mutations can result in decreased susceptibility to d4T, however, should not lead to the conclusion that d4T treatment failure can always be explained by this novel pathway. [3] As a matter of fact, the overall frequency of this second multi-nucleoside resistance pathway may be very low. In this regard, the majority of the cases reported at the Workshop were actually selected for by combination nucleoside therapies that did not include d4T. In all of the cases, patients were initially treated with AZT, with a second nucleoside (ddI, ddC) being added at a later point. It remains to be determined whether pretreatment with AZT is a prerequisite for the development of the codon 69 insertion pathway.

Similarly, predisposing factors for the emergence of multi-nucleoside resistant viruses, by any mechanism(s), need to be investigated further. A European survey of the incidence of the first described multi-nucleoside drug resistance pathway, performed by participants of ENVA (the European Network for the Virological evaluation of international trials on new Anti-HIV therapies), was reported. [4] This pathway involves a change at position 151 of HIV-1 RT, and a total of five additional changes (A62V, V75I, F77L, F116Y and Q151M), which appear to confer in vitro cross-resistance to nucleoside analogues as a class. Taking into account that the change at codon 151 is the first mutation to emerge, this mutation was used to screen 350 samples from patients treated with two nucleosides for at least 6 months. Seven patients (2%) were identified with a Q151M change. Again, all patients had received AZT in combination with ddI, ddC or d4T.

Currently, there are no large cohorts on treatment with nucleoside combinations that do not include AZT. Therefore it is not known whether AZT therapy is a prerequisite for the development of Q151M MDR, or whether other nucleosides may also contribute to this mechanism. In addition, we do not have clinical data addressing the clinical relevance of either of the two MDR pathways discussed above. However, it can not be excluded that individuals developing multi-nucleoside resistant viruses (by either the Q151M or 69 insertion pathways) may have limited virologic responses to changes in their nucleoside regimens.

Clinical Utility of Drug Resistance Testing

In a session entitled "Relationship Between Phenotype, Genotype and Clinical Response," several presentations were made in which the response to therapy (as signified by HIV RNA and CD4 changes) was analyzed in relation to the development of drug resistance.

Cohort analyses of patients receiving AZT+ddI+nevirapine (ACTG 241) showed that three baseline sequence clusters, which had resulted from AZT pretreatment, were associated with differences in response to treatment after 48 weeks of therapy. [5] Similarly, the response to abacavir seems to be dependent on the viral genotype at the time of treatment initiation. Individuals with multiple mutations to several nucleoside analogues (including AZT and 3TC) seem to have an attenuated or no response to abacavir. [6]

These data, in addition to reports on the relationship between the development of PI resistance and varying responses to therapy after switching PIs (based on the level of protease inhibitor resistance and/or the type of changes detected in the protease enzyme), generated a lively discussion on the usefulness of resistance testing in the clinical management of HIV infection.

From all the data presented, it seems that some guidance on the choice of a new regimen for patients who are failing another regimen can be provided by genotypic and/or phenotypic information on drug resistance. However, it is also obvious that the value of these measurements in clinical practice could be greatly improved by expanding current databases and by performing prospective studies designed to evaluate the clinical value of resistance measurements. Although phenotypic tests are becoming more widely available, the costs and workload involved in performing these assays remain limiting factors for their widespread clinical use.

Since many diagnostic companies are focusing their efforts on providing commercially available genotypic assays to virology laboratories, it is quite likely that these assays will become the standard for clinical resistance testing, especially because it will become increasingly important to be able to detect the development of drug resistance earlier and earlier in patients with relatively low amounts of virus. A critical point, however, is that good databases need to become available to guide the development of treatment algorithms that address specific genotypically-identified resistance scenarios.

Primary Infection with RTI-Resistant HIV-1

Numerous studies were presented on the prevalence of drug-resistant variants in untreated individuals from different parts of the world and different risk groups. [7-13] Both in the USA and in Europe, a prevalence of 5-8% was reported for mutations in the HIV-1 RT gene. Interestingly, in one longitudinal study from 1993-1998, a peak incidence was found in patients presenting to the clinic in 1995. [10] A follow-up of patients infected with viruses containing AZT resistance mutations showed that these changes could persist for as long as four years after infection in patients not taking any antiretroviral therapy.

Two studies identified cases of transmission of truly MDR isolates. [7,8] Two cases, one in San Francisco and one in Switzerland, were identified where virus obtained at the time of primary infection harbored changes in both the protease and RT genes. The San Francisco case was particularly interesting since the newly-identified patient was put on triple therapy shortly after becoming infected. The virus from this patient had most of the classical AZT changes as well as the 184V mutation. In addition, there were changes in the protease gene at codons 82, 88 and 90, as well as at several other positions. Careful analysis of the patient's viral RNA response to therapy indicated reduced activity of the triple therapy in this patient. This indicates that therapeutic options may be limited in cases of de novo infections by MDR isolates. It also raises the important question of whether we should start testing for resistance before therapy is initiated.

Whereas we are currently lacking larger surveys on the incidence of transmission of drug-resistant viruses, the importance of the above reports is that they show that such transmissions apparently do occur. It therefore seems important to develop monitoring systems that can generate insight into the incidence of transmission of MDR HIV, and into possible predisposing factors for this type of transmission.

Resistance to Non-Nucleoside RTIs

A limited number of reports was presented on the development of resistance to NNRTIs. Analyses of patients failing combinations containing efavirenz showed that the most common mutation associated with treatment failure was K103N. [14] This change was observed in the vast majority of patients with treatment failure, and in most cases was accompanied by V108I and P225H mutations. Additional changes previously reported to occur with NNRTI therapy at positions 100, 188, 190 were also seen in some patients.

Reconstruction experiments showed that an increase in IC₅₀ of approximately 17-fold conferred by the K103N mutation is greatly enhanced (>100-fold) by the addition of V108I of P225H mutations. In one patient, combination of the L100I and K103N mutations led to an IC₅₀ increase of more than 4000-fold.

Related HIVresistanceWeb Articles:

Report From the Second International Workshop on Drug Resistance and Treatment Strategies (24-27 June, Lake Maggiore, Italy): Protease Inhibitor Drug Resistance
(Douglas Mayers, July/August 1998)

References

.Phenotypic and molecular analysis of HIV-1 isolates possessing 6 bp inserts in the reverse transcriptase gene that confer resistant to nucleoside analogues.  Winters MA, Cooley KL, Girard YA, Levee DJ, Hamdan H, Katzenstein DA, Shafer RW, Merigan TC.Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 16.

Insertion of two amino acids in reverse transcriptase (RT) during antiretroviral combination therapy: implications for resistance against nucleoside RT inhibitors.  De Jong JJ, Jurriaans S, Goudsmit J, Baan E, Huismans R, Danner S, Hillebrand, ten Veen JH, de Wolf F.Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 18.

Virological basis for HIV-1 resistance to stavudine investigated by analysis of clinical sample.  Bloor S, Hertogs K, Desner RL, Pauwels R, Larder BA. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 15.

Prevalence of multinucleoside drug resistance among European HIV-1 infected patients receiving various combinations of nucleoside analogues.  Van Vaerenbergh K, Van Laethem K, Albert J, Boucher C, Clotet B, Floridia M, Nielsen C, Pedersen C, Perrin L, Pirillo MF, Ruiz L, Schmidt JC, Schneider F, Schoolmeester A, Schuurman R, Stellbrink HJ, Stuyver L, Van Lunzen J, Van Wijngaerden E, Vella S, Yerly S, De Clerq E, Desmyster J, Vandamme A-M. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 69.

Baseline sequence clusters predict response to combination therapy in ACTG 241.  Leigh Brown AJ, D'Aquila RT, Johnson VA, Kuritzkes DR, Richman DD. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 50.

Genotypic and phenotypic correlates of response to abacavir (ABC, 1592).  Lanier R, Danehower S, Daluge S, Cutrell A, Tisdale M, Pearce G, Spreen B, Lafon S, Kemp SD, Bloor S, Larder BA. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 52.

Reverse transcriptase and protease gene analysis at the time of primary HIV-1 infection.  Yerly S, Kaiser L, Race E, Clavel F, Perrin L. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 107.

Transmission of HIV-1 resistant to multiple reverse transcriptase and protease inhibitors.  Grant RM, Hecht FM, Petropoulos CJ, Dillon B, Chesney M, Tian H, Hellmann NS, Kahn JO. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 108.

Prevalence of drug-resistance mutations in the HIV-1 reverse transcriptase and protease region in previously untreated patients from Belgium.  Verhelst R, Verhofstede C, Van Der Gucht B, Van Wanzeele F, Rossau R, Stuyver L. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 113.

Genotypic analysis of HIV-1 pol genes from drug-naïve patients presenting to clinic between 1993-1998 and drug experienced patients failing antiretroviral combination therapy with reverse transcriptase inhibitors and protease inhibitors.   Kozal M, Leahy N, Hanranhan J, Swack N, Stapleton J. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 114.

Prevalence of HIV-1 drug resistance mutations in antiretroviral-naïve patients at an inner city University of Miami HIV clinic in Miami, Florida, USA, in 1997.  Kozal M, Leahy H, Agar A, Jayaweera D. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 115.

Antiretroviral drug-resistant genotypic mutations in HIV-infected persons at first diagnosis (1988-1997).   Williams JG, Kaye S, Williams DI, Bennett J, Tedder RS, Weller IVD. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 116.

Prevalence of drug-resistant HIV-1 variants of RT region and natural polymorphism in protease coding region of recently infected individuals.  Riva C, Berlusconi A, Violin M, Colombo MC, Corvasce S, Gori A, Gervasoni C, Rusconi S, Mazzucchelli R, Galli M, Moroni M, Balotta C. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 117.

RT gene mutations associated with resistance to efavirenz.  Bacheler LT, Anton E, Jeffrey S, George H, Hollis G, Abremski K, Sustiva Resistance Study Team. Second International Workshop on HIV Drug Resistance and Treatment Strategies. 24-27 June 1998, Lake Maggiore, Italy, Abstract 19.

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