Report 14 -  4th International Workshop on HIV Drug Resistance and Treatment Strategies
Written by Jules Levin
Sitges, Spain, June 12-16 2000

What is Viral Fitness?


It has been found that CD4s can remain elevated for many individuals even after viral load increases or rebounds when a person is taking HAART. This was first reported 1-2 years ago by Dr. Steve Deeks. It's generally accepted that remaining on a virologically failing regimen (when viral load is detectable) permits for the accumulation of resistance mutations. This can cause cross-resistance to all drugs and prevent a good response when switching to new drugs. Therefore, I think it's generally accepted today that if a person has good treatment options they should probably switch therapy as quickly as possible. However, newly developing thinking is that--due to CD4s remaining elevated, and if a person with detectable viral load has few or no treatment options, and if viral load is not too high, it's being suggested that they may be better off staying on the same therapy or possibly it's speculated some "maintenance" therapy (maintaining mutations and reduced viral replication capacity). Research findings suggest that when resistance mutations are present a person's virus may not be as harmful because it may not be able replicate as easily. Please read the full report for a better understanding of the underlying concepts.


What is viral fitness? Viral fitness or replication capacity, as some say is a more appropriate term, is becoming a focus of attention. It received a good deal of attention at the Resistance Workshop in Sitges and 2 papers addressing it are reported below. Since you will likely hear increasing information and research focused on this topic it's important to understand the term and it's clinical applicability. This is what I hope will be the first of several reports on this topic.

As you may know, Steve Deeks was probably the first to report 1-2 years ago I think at ICAAC that although plasma viral load increases in individuals experiencing virologic failure on HAART, CD4s can remove elevated. Although we may not have appreciated the full impact of these initial findings back then, researchers and doctors continue to explore it's implications. If a person experiences virologic failure following their first HAART regimen or they have adequate treatment options, it may be important to that patient to switch their regimen as quickly as possible. This would minimize drug resistance and help to reserve future response to therapies. Remaining on a regimen while viral replication is ongoing is likely to result in the accumulation of resistance mutations and cross-resistance to other drugs in the same class. However, what about the person who has few or no feasible treatment options? Or possibly you may want to save the few options available until they can be optimized when new treatments become available. In the near future, new drugs which may help treatment for individuals with resistant virus include T-20, DAPD, ABT-378, tipranavir, and PMPA.

If CD4s can remain elevated while viral load is detectable or increasing, is it possible that detectable viral load at some reasonably safe level may not matter that much? And if so, for how long can it be detectable and still be safe, and what level of viral load is safe? These kind of questions raise questions about treatment approaches and guidelines? Maybe some level of detectable viral load is ok. Of course, there are many questions regarding safety and long term affects of such an approach. The dogma for several years has been that viral load must be retained to as low a level as possible, preferably <50 copies/ml. This still seems to me to be the preferred goal of therapy. But the ability for CD4s to remain elevated despite detectable viral load raise many questions about how to apply treatment. Since the beginning of the new age of HIV treatment which emerged with the advent of protease inhibitors, and as we acquire increasing experience, treatment is becoming increasingly complex and maybe new approaches will emerge.

At the June 12 2000 Sitges 4th International Workshop on HIV Drug Resistance & Treatment Strategies John Coffin, a noted researcher, said viral fitness is the wrong term. It should be called replication capacity. Here are 2 interesting papers presented on this topic at Sitges.

Drug resistance is associated with impaired protease and reverse transcriptase (RT) function and reduced replication capacity

Terri Wrin of Virologic, Inc. reported early findings from an ongoing in vitro (in the test tube) study of the potential correlation between drug resistance, replication capacity and the biochemical fitness of RT and protease. They evaluated recombinant viruses from randomly selected patient samples (over 100 samples) submitted to Virologic for phenotypic testing. It's believed that mutations conferring reduced susceptibility also impair viral replication (making a virus possibly less virulent), which can in part be restored by additional compensatory mutations. But data to support this hypothesis is needed.

Wrin reported that recombinant viruses containing patient derived protease and RT genotypic sequences (mutational patterns) exhibited a broad range of replication capacity (<1% to >100%) when compared to a well characterized reference strain (NL4-3). Wrin reported that a strong correlation was seen between drug resistance and reduced replication capacity. About 85% of the viruses with impaired replication capacity ranging from 1 to 30% of the reference virus showed high phenotypic resistance to antiretroviral drugs. As well, low replication capacity was most often associated with inefficient protease cleavage at several sites in the Gag and Gag-pol polyproteins. Findings indicated that specific mutations impaired cleavage at distinct sites. Wrin reported that mutations D30N, M46I/L, G48V, I54L/A/S/T/V, and I84V correlated strongly with Gag processing defects. She reported that nelfinavir resistant viruses in particular, exhibited many protease cleavage defects and 70% of nelfinavir-resistant viruses showed large reductions in viral replication. Low replication capacity was associated with the number of protease mutations and the presence of either the nelfinavir-resistance mutation D30N alone or L90M in combination with mutations at 20, 46, 73, or 88.

A small number of viruses (5%) with impaired replication capacity also exhibited hypersensitivity to protease inhibitors. I think this relates to the separate in vitro observation that the N88S mutation in the presence of nelfinavir or indinavir resistance mutations increased amprenavir susceptibility (2.5- to 12.5-fold) with 20 of 312 (6.4%) patient viruses analyzed. This was also reported by Virologic researchers (Journal of Virology, May 2000, p. 4414-4419, Vol. 74, No. 9).

In contrast to findings related to protease, less than 10% of the viruses with low replication capacity displayed a measurable defect in virion-associated RT (less than 20% of wild-type levels) even though RT inhibitor resistance was common. Wrin concluded that resistance to protease inhibitors has a more deleterious effect on viral replication capacity than RT inhibitor resistance.

Impact of NRTI Mutations on HIV-1 CTL (cytotoxic T-cells) Responses

Assia Samri from the lab of Bridget Autran in Paris reported on a study that I think bears relevance to reduced replication capacity or viral fitness. The purpose of this study was to determine the impact of drug-resistance mutations appearing during NRTI therapy on CTL recognition of the RT enzyme. Two trunctated regions of the RT enzyme (RT-1: 1-143; RT-2: 143-293), containing sites of mutations, were tested on Pol-specific CTL lines for 2 patient groups-those treated with mono- or bi-therapy with NRTIs. Mutations M41L, L74V, M184V, and T215Y/F were evaluated in these patient's samples by LiPa. Overall, CTL recognition of wild-type RT-I or RT-2 was comparable in treated and untreated patient samples, but twice as frequent in the samples from patients who had been NRTI treated and had NRTI mutations. RT-1 was recognized in 83% of treated samples containing the M41L and/or L74V but in only 42% of samples without mutations. Similarly, RT-2 was recognized in 75% of samples containing mutations M184V and/or T215Y/F, but in only 33% of samples without mutations. Amongst those, NRTI-induced mutations enhanced HLA-binding scores in 17 cases (42%), decreased them in 5 cases (12%), while scores remained unchanged in 19 cases (46%). Four of 5 predicted epitopes were recognized at least once in an ELISPOT assay. The frequencies of IFN-y sot-forming cells were between 40 and 270 per 10(6th) PBMC, similar to known CTL epitopes in RT.

Samri concluded that RT mutations induced by NRTIs rarely decrease but can increase the immunogenicity of RT for CTL recognition and might allow a better immune control of resistant viruses.

I think it's important to conduct clinical studies in HIV-infected individuals to explore the many possibilities implied from these findings and concepts outlined above. Maybe a low level of detectable virus is safe over the long haul. Previous clinical research findings have suggested that viral load of 5,000 copies/ml or less may be safe. Well designed research into these questions may result in very important findings which could profoundly change and improve treatment approaches.

PERSONAL EXPERIENCE & SPECULATION. As an example of how this concept of reduced replication capacity may apply in real situations I'll relate my experience. I started treatment with AZT alone about 7-8 years ago, and added 3TC to AZT about 2 years following the start of AZT treatment. I assume I had detectable viral load throughout but I don't know because viral load testing was not available to me yet. When I started testing my viral load, several months before switching to a HAART regimen, the levels were relatively low and remained relatively stable although I assume I had resistance mutations. Recently, I performed a genotypic resistance test while my viral load was <50 copies/nl and found an AZT mutation (I think it was the 215) and the 3TC mutation (M184V). I assume I had those mutations while on AZT/3TC alone. The point is that my CD4 count remained stable throughout the entire approximate 5 years on AZT monotherapy and AZT/3TC dual therapy, after it was taking a precipitous decline just prior to starting AZT therapy. Possibly, NRTI mutations reduced the replication capacity of my virus and permitted my CD4s to remain stable.