Antiretrovirals and Resistance

So this is a big topic, and I had put together a PowerPoint (with the "script" in the notes section beneath each slide) for a School of Pharmacy class in 2008 on antiretroviral resistance testing. It's at a level that I think is good for people getting acquainted with the principles of resistance tests:

http://www.unc.edu/~churt/downloads/ARV_Resistance_Hurt.ppt

This covers the basics of:

• why HIV mutates so readily
• principles of the techniques involved for resistance testing
• who needs to get a test (by the 2007/08 DHHS guidelines)
• what happens to your patient's blood sample once you order a test
• the differences among phenotypic, genotypic, and "virtual phenotype" testing
• a tangent on why certain positions (like K103N or M184V) are so significant, but other positions aren't (the answer is tertiary protein structure)

I'd welcome any questions or comments on this. The top 5 gurus for ARV resistance here (in order of who you should talk to if you have a significant, burning question) are:

1. Joe Eron
2. Joe Eron
3. Joe Eron
4. David Margolis
5. David Wohl

The thumbnail version
Basically, HIV mutates so quickly because it's crappy at proofreading itself. The reverse transcriptase (RT) has poor (but present) proofreading activity, so if it mismatches a nucleotide base-pair, it can't stop, back up, and excise the mismatch. Instead it blows through the copy, generating many, many point-mutations in the sequence of each copy. Poor fidelity in the copying process means more mutations, some of which are fatal to the protein/enzyme, rendering it useless - but others are fortuitous and confer resistance to a medication. The selection pressure of the ARVs drives natural selection in a beautifully Darwinian way. This is an important point, because without the selection pressure, the most "fit" (able to copy itself readily) version (aka "quasispecies") of HIV in the body will overgrow all others. This is usually referred to as "wild-type," meaning there's no native resistance. However, keep in mind that if a patient is infected with resistant virus at the outset (transmitted drug resistance), then that is their wild-type. Some mutations will revert back towards wild-type over time, through back-mutations, but others may persist for months-t0-years after acute infection. This is the underpinning for why the DHHS recommends now that all patients newly entering HIV care have some baseline assessment of antiretroviral resistance.

At UNC, our test of choice is the "virtual phenotype" or VIRCOtype, provided by Virco, a company with an office in the RTP but whose main offices (and labs) are in Belgium. This test combines a repository of straight-up nucleotide sequence data with one of phenotypic resistance information as lab-cultured viruses are exposed to drugs and their replication measured. Although it has some warts, it's pretty good at predicting what meds will work and which won't. It can't predict synergy, however, and that's a significant drawback.

The mutations are alphabet soup and will be for a while. Don't freak out, it's supposed to be that way – and I never feel bad about looking at one of the IAS-USA resistance cards in the clinic, since even people like Joe Eron and Amanda Corbett refer to them routinely. Realistically, you don't need to memorize any mutations. The ones that make a difference will be repeat customers and you'll come to know them by repetition. Others are less important and you have other stuff to fill your brain up with as things get going for you here. (Plus as each new med comes out of the pipeline, a new set of mutations will become apparent as more and more people get on them.)

– Christopher Hurt