By David N. Leff
Sooner or later, it had to happen.
Since 1987, when zidovudine (AZT), inhibitor of HIV replication, entered clinical practice to treat HIV-infected patients, their physicians have anxiously wondered how long before the virus caught up with AZT, and the new triple-drug cocktail of HAART, or highly active antiretroviral therapy.
Now it looks as if the human immunodeficiency virus is training its drug-resistance guns against zidovudine, one of the cocktail¿s star performers. The viral pathogen fired a shot across the bows of 603 treatment-naove, recently diagnosed HIV-1-infected patients enrolled between 1997 and 1999 at clinical centers in 10 American cities.
¿This study was designed,¿ observed virologist J. Gerardo Garcia-Lerma, ¿to evaluate prevalence of viruses with resistance mutations conferring drug resistance.¿ A visiting scientist at the Centers for Disease Control and Prevention (CDC) in Atlanta, Garcia-Lerma is first author of a paper in today¿s Proceedings of the National Academy of Sciences (PNAS), released Nov. 6, 2001. Its disquieting report is titled: ¿Increased ability for selection of zidovudine resistance in a distinct class of wild-type HIV-1 from drug-naove persons.¿
¿We found,¿ Garcia-Lerma told BioWorld Today, ¿that there was substantial prevalence of virus having a unique set of mutations in their reverse transcriptase gene. These viruses, which we detected in 20 of the 603 patients ¿ 3.3 percent ¿ showed increased ability to become resistant to AZT. This distinct class of viruses was different from the common wild-type HIV. What we saw in vitro is that, with these mutations, it can develop drug resistance more rapidly than the commonly found viruses.
¿When we screened for resistance mutations in this large patient population,¿ he went on, ¿we found some harboring virus that had resistance mutations ¿ the common ones, but also viruses sensitive to zidovudine, but able to develop resistance factors in culture. Now it¿s known that 5 [percent] to 10 percent of the newly diagnosed may harbor viruses with a resistance mutation. And there are studies ongoing to evaluate whether this prevalence will increase, or not, over time. Based on that, we should evaluate the strategies to decrease the problem of transmission.¿
Main Threat: Passage From Virus To Virus
¿Transmission of HIV-1,¿ the PNAS paper begins, ¿with reduced susceptibility [sensitivity] to antiretroviral drugs raises public health concerns.¿
Garcia-Lerma picked up on this theme: ¿It¿s not been known before, but this unusual mutation is transmissible; it has already happened. It was published a couple of months ago on one or two patients who were infected with this intermediate mutation. But what we don¿t known in our patient population is whether these mutations were transmitted or these persons were infected with a mutant virus, which reverted.¿
He made the point, ¿These viruses can replicate efficiently ¿ and become reverted for one to three years in a patient who is not being treated with drug. We don¿t know the clinical impact of this mutation. Whether or not this is going to influence treatment ¿ the sensitivity ¿ is to be addressed in future studies.¿
Garcia-Lerma explained the molecular difference between these oddball, 3.3 percent mutations and the run-of-the mill ones:
¿We see a particular codon of the reverse transcriptase gene, and this is codon 215. Its usual mutations are two amino acids, tyrosine and phenylalanine. But this class of unusual viruses has a different set of mutations ¿ mainly aspartic acid and cysteine. These represent revertants ¿ mutants that have returned to their former genotypes. That is, the mutant in the absence of AZT, goes back to being a wild-type virus.
¿In this reversion,¿ Garcia-Lerma pointed out, ¿there are some intermediates that we found in this patient population. If you need two base-pair changes in the genome to go from wild type to mutant, intermediate means only one base pair change to the mutant form. And this is what happened here. These intermediates have been shown to be stable; they can persist for years in the absence of drug.
¿Another interesting thing,¿ he added, ¿is that these intermediates are sensitive to AZT, not resistant. So they have the same characteristics as the wild-type virus. What distinguishes these intermediates from the wild type is they can evolve very fast in culture to the mutant form.
¿What needs to be done next,¿ Garcia-Lerma pointed out, ¿is to evaluate whether this mutation can influence the efficacy of retroviral treatments such as HAART, containing AZT. That¿s essential and urgent. We are now addressing the ability of this virus to evolve, to mutate, in the presence of a different drug ¿ stavudine. It, too, is a reverse-transcriptase inhibitor, like AZT. They have different mutations in some cases. But in other cases they select for the same T215Y mutations. That is the amino-acid chain expressed by the reverse-transcriptase genome,¿ he explained. ¿The letter T, representing the wild-type virus, is threonine at position 215 ¿ and then that mutates to Y ¿ tyrosine.¿
HAART Regimen Unthreatened ¿ For Now
The PNAS paper noted: ¿The T215Y is a primary mutation observed after AZT treatment. T215Y alone reduces the susceptibility for AZT approximately 16-fold and is the first mutation seen in the majority of patients receiving combination therapy with AZT and other [HAART drugs].¿
¿So a person can be infected with a virus carrying the T 215 Y mutation,¿ Garcia-Lerma pointed out, ¿but if you hunt for the mutant virus, which is circulating in that person out there for a period of time after infection, you may not find it in plasma. But it¿s in the cells. It may be archived, which means it is present but you cannot detect it circulating. This is also referred to as fossil virus.¿¿
The looming threat of an all-out drug-resistance attack against AZT by HIV does not signify any need to modify the current HAART. ¿At least not at this moment,¿ Garcia-Lerma opined. ¿Rather,¿ he urged, ¿what is prudent is to report these viruses with these intermediate mutations ¿ to know that they may occur, may be there, and there is a need to inform the clinician, who can then decide whether it is advisable to report these mutations, which may compromise difficulty of treatment. Since we see that these viruses can evolve very rapidly to the mutant form, it is prudent to report these intermediate mutations in genotypic test results.¿