Inhibition of reverse transcriptase

[Patrick O'Neil]

On 19 May 1995, Andrew, Tel. +396-91093434 wrote:

>    Yes indeed, it is possible to find compounds which inhibit reverse
> transcriptase and in particular much effort (and money) has been expended over
[...]
> along these lines?) Many of these compounds function very well in the test
> tube but beyond that there are many difficulties in using these on people,
> usually because of one or more of the following problems:
> 
> 1) The compound is too toxic to be used safely in treating people with HIV.
> 
> 2) Treatment with even safe and "test tube effective" compounds results in the

[...]
> rapid appearance of mutant HIV strains which are resistant to the drug.
> with a variety of different protease inhibitors. In all cases drug-resistant
> strains of the virus eventually emerged which were not only resistant to the
> original drug used on the patient but were also cross-resistant to the other
> anti-protease drugs tested.
> 
>    They are now hoping that treatment with combinations of drugs having
> different activities will be effective. I hope so.
> 

Theoretically, if enough different (largely unrelated by structure or 
specific function) chemicals/drugs are developed with antiviral activity, 
such that treatments with one constellation of agents can then be 
replaced with another, etc, it should be effective.  

Treatment with AZT and a specific protease inhibitor, for instance, will 
invariably lead to selection of resistant strains.  If there are other 
drugs available that are only loosely related to the previous treatments, 
then hopefully, the mutant/resistant variant will not carry a resistance 
to the next treatment constellation.  A different base analog chain 
terminator, along with another drug targeting either another point on the 
protease or targeted at another protein component should require 
completely independent mutation events from the first in order to get 
resistance.  Continuing this chain over completely independent drugs 
could provide sufficient time for the original AZT-protease resistant 
strain to essentially disappear from the immediate viral population 
(selection for other variants and taking advantage of the intrinsic low 
fidelity of RT:  lack of selective pressure would allow mutation away 
from the first resistant genotype).  At this vague point, the original 
drug treatment could be started all over again, and so on.

One of the ultimate goals of the lab I work in, regarding retroviruses, 
is to find ways of increasing the error rate of RT.  As I mentioned, it 
is already very error-prone to the point that if the error rate were 
increased a bit more (a grey area), it would be so error-prone that the 
majority of virions would be non-infective, allowing for the immune 
system to catch up and overtake the virus.  That is the idea, at any rate.

One neat in vitro trick I read about earlier this year (I'll have to find 
a reference) involved using a retroviral vector to insert a gene encoding 
an RNAse fused to an HIV packaging signal.  The HIV infected cells 
treated thus in culture, demonstrated about a 90% reduction in productive 
virus.  Virions were released but most of them were dead-end...the RNAse 
was found to properly localize to the forming virions where it began 
hydrolyzing the HIV RNA genome.  There was no measureable affect on 
normal, cellular RNAs.  This is nice in that it doesn't rely on hitting 
an enzymatic activity of the virus, so that the  virus cannot evolve a 
resistance (short of giving up its RNA genome or completely changing its 
packaging signal).  The downside is that it requires a sort of infection 
with another retrovirus, strictly as a vector, and consequent random 
integration into the host cell genome which can lead to other problems, 
though very rare.    

In any case, I like it.

Patrick