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T-20 and Beyond: Inhibition of HIV Attachment and Fusion at the 9th CROI
Reported by Christopher D. Pilcher, M.D.
UNC at Chapel Hill Center for AIDS Research
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THE NEED FOR NEW DRUG CLASSES
One of the chief obstacles currently facing patients with previous heavy treatment experience is the fact that all available agents work in similar ways, interfering in one way or another with one of two viral enzymes: HIV reverse transcriptase (RT)-which the virus uses to replicate itself within the cell, and HIV protease-which the virus uses to package itself for export out of an infected cell, to begin new rounds of destruction. The similar modes of action mean that when the virus adapts to one drug's presence by developing genetic resistance mutations, these mutations can impair the activity of some or all of the other drugs in that drug class (a phenomenon called "cross-resistance"). As resistance mutations accumulate, patients' future treatment options quickly shrink.
In contrast to RT and protease, the processes by which the virus mounts its initial assault on the cell membrane (virus entry) are not assailable by any current drugs. For this reason, treatments targeting these points in the viral life cycle would be expected to have excellent activity against even highly protease or RT inhibitor-resistant strains of HIV. At this time last year, there was a widespread enthusiasm concerning developments in the quest to develop medicines targeting viral entry. At the 9th Conference, the difficulties of moving these compounds forward have become more apparent and the celebration has become muted.
VIRUS ENTRY: ATTACHMENT, THEN FUSION
The events of virus entry into cells occur in several steps: the first step in the HIV-cell interaction that leads to virus entry is "attachment", followed by "fusion" that is necessary for entry into the cell:
The virus' gp120 surface molecule fits into the CD4 receptor on the cell surface like a key in a lock and the virus becomes attached-but is left hanging loosely from the cell surface by this gp120-CD4 connection.
Once it is attached, gp120 is bound to another receptor (either one of the "co-receptors" for gp120, called CCR5 or CXCR4). Thus doubly bound, the gp120 molecule flips around to expose HIV's previously hidden "harpoon" molecule, gp41.
Once the free end of gp41 spears the cell membrane, solidifying HIV's attachment, gp41 doubles itself up into a tight coiled-coil structure. This brings the two ends of gp41 (one holding HIV, and one holding the cell membrane) very close together. The cell membrane and virus envelope come into contact and fuse. Fusion results in the virus' contents getting dumped into the interior of the cell. Virus 1, Cell 0.
POTENTIAL ADVANTAGES TO ENTRY INHIBITORS
It is important to remember that all of these processes occur on the cell surface, so that drugs targeting them can work "outside the cell" without any requirement for intracellular accumulation or metabolism. It is thus possible that entry inhibitors could evade possible "cellular resistance". Different from viral resistance, cellular resistance can occur when host cells perceive drugs as toxins and adapt by pumping them quickly back out into the extracellular space. This has been proposed as one potential explanation for why potent HAART sometimes fails to control virus replication in patients-and might thus represent a potential additional advantage to using entry inhibitors.
It is also likely that "synergy" could be seen among the different classes of molecules targeting HIV entry when used together. As first explained by Robert Doms at last year's Retrovirus conference, by delaying the process of attachment and fusion, coreceptor or CD4 antagonists can keep the uncoiled gp41 molecule exposed for longer than normal-opening up this target to attack by gp41 blockers like T20 or T1249. This provides a theoretical basis for hope that an even higher level of potency can be obtained by fusion and entry inhibitors in the future, and that these so far troubled drugs may have an important place in HIV treatment.
POTENTIAL HAZARDS
Researchers have traditionally been wary of any drugs blocking CCR5 in particular because although viruses usually start by using CCR5 preferentially, they tend to mutate to use CXCR4 around the time of the onset of clinical AIDS. The fear has been that by purposely blocking CCR5 use, the so-called "X4 switch" could be induced and patients hastened on the road to AIDS. At least in the few mice and humans treated with CCR5-blocking drugs so far, this has not been shown to happen, but remains a central concern.
There are other unanswered questions about general safety for coreceptor (CCR5 and CXCR4) blockers. Coreceptors themselves are extremely common on the surfaces of all kinds of cells in the body-not just infected cells, but healthy cells. They may play key roles in embryogenesis and proper immune function as well as numerous other body processes. The consequences of blocking these receptors for the body are not known and an extra note of caution is probably warranted (see notes on SCH-C, below).
FUSION INHIBITORS: T-20 and T-1249 CHUGGING AHEAD
For inhibitors of fusion such as T-20 and T-1249, no such general safety concerns have been forwarded. These compounds are protein molecules (T20 has 36 amino acids) that basically bind to gp41 and prevent it from coiling up to reel the cell and virus together-preventing fusion. With T-20 nearing completion of its phase III development and T-1249 showing no red flags in phase II, the availability of these drugs to patients sometime soon seems likely. And although both agents have shown significant activity with little evidence of systemic toxicity in clinical trials to date, they are seriously limited by the fact that they are large peptides that cannot be absorbed from the gut. They must therefore be injected (twice daily) and can result in sometimes troubling injection site reactions.
In a 48 week update of T20-206 (Abstract 418-W)-a study comparing abacavir, ritonavir-boosted amprenavir and efavirenz to the same regimen + 3 separate doses of T-20 in a total of 71 patients failing PIs at baseline-Roche-Trimeris presented data confirming previous impressions. Over 48 weeks, approximately 70% of subjects across T-20 arms experienced injection site reactions. Of the subjects with reactions, about half in each T-20 arm reported that these were mild; half reported at least some that were moderate in severity or worse (only 3 of 54 T-20 treated patients discontinued the drug as a result). So while the tolerability of long term T-20 for widespread clinical use is an open question, there were still no notable differences in systemic toxicity between the T-20 and no-T20 groups over the full 48 weeks of T-20-206, and activity of the combinations with or without T-20 reported last year at 16 weeks-slightly favoring the T-20 arms-appeared sustained over 48 weeks.
(editorial note: patients receiving the non T20 regimenin that study had 37% <400 copies/ml and 37% <50 at week 48; patients in the 3 T-20 dose arms had 55% <400 and 47% <50. The T20 group appeared to sustain benefits better than the non T20 group. In the T20 group, 69% had <400 and 35% <50 at week 12 compared to 55% and 47% respectively at week 48. In the non T20 group 58% had <400 and 32% had <50 at week 12, compared to 37% <400 and 36% <50 at week 48. In the 3 T20 dose arms the viral load reduction was -2.24 log at week 12 and it was -2.24 at week 48, showing a sustained result. But in the non T20 group the viral load reduction was -2.13 at week 12 and -1.87 at week 48. The CD4 increase was 132 in the T20 group vs 90 in the non T20 group. This data was not powered for comparison so the differences may not be significant. As well, the poster containing this data did not specify what type of analysis was performed, if results were on-treatment or intent-to-treat).
ATTACHMENT INHIBITORS
Focus at this year's conference shifted to the progress in clinical development of two candidate attachment inhibitors that block the attachment of HIV gp120 to CCR5 or CXCR4. These were first introduced to a general audience at last year's conference: Schering-Plough's SCH-C and AMD-3100 from AnorMed.
CCR5: SCH-C TOXICITY
Unlike most other attachment and entry inhibitors, SCH-C is a small molecule that has excellent absorption from the gut (60-90% bioavailability), low protein binding and a prolonged effect after ingestion (~25 hours) in addition to potent antiviral properties. SCH-C had previously shown potent activity in its first phase I trial but had also been noted to cause significant QT prolongation (an electrocardiographic abnormality that, when extreme, can herald life-threatening cardiac arrhythmias). In an oral session on Monday (Abstract 1), M. Laughlin from the Aaron Diamond AIDS Research Center presented detailed results of another phase I experiment in using a range of lower doses of the drug that was somewhat if not entirely reassuring. The doses tested delivered 25 to 200 mg every 12 hours (as compared to the 400mg/day that was toxic in the first trial) to 12 ART-naive patients with NSI-or CCR-5 using-virus. Treatment was monotherapy. Laughlin reported data on the initial low dose group (25 mg). Activity was potent over 10 days with 10/12 patients achieving a 0.5 log (3-fold) drop in viral load and 4/12 dropping more than 1.0 log, or 10-fold. Two patients had no response at all, for reasons that were not clear. Tolerability was better than at higher doses, however 3 patients had headache and 2 complained of bad taste. Most significantly, there was consistent and progressive QT prolongation, to a mean of 11 msec at 10 days of therapy. While this degree of prolongation did not cause the trial to be prematurely terminated for safety, it is clinically meaningful and might represent a serious problem for this particular drug to go forward. These results were taken to indicate that CCR5 is a "valid target" for inhibition; however the potential safety concerns remain very troubling and are very likely to limit progress of this particular compound. (editorial note: the company developing this drug, Schering Plough, says they are proceeding ahead to the next study and so far do not see a safety problem indicating stopping development of SCH-C. Schering has another drug, SCH-D, a backup drug for the C drug).
Another CCR5 blocker named PRO 140 is an antibody molecule that specifically targets CCR5, and showed impressive activity in a mouse model; some Merck compounds are also undergoing preclinical development and the results of these studies will be the subject of future meetings.
CXCR-4: AMD-3100 STUMBLES
Among the CXCR-4 inhibitors, the granddaddy of compounds is AnorMed's AMD3100, a bicyclam compound that had appeared highly potent in pre-clinical studies but like T-20, has to be given by either subcutaneous or intravenous injection. In a poster presentation (Abstract 391-T), Hendrix and colleagues presented data from a trial in which 40 subjects with stable or no ART were treated in a dose-ranging study to a maximum dose of 160 mcg/day. Antiviral effect was minimal, with no patients reaching the 1.0 log drop definition of successful activity and only 5 having a greater than 0.5 log drop. Despite this, side effects were significant with a majority of patients in all cohorts developing numbness or tingling, atrial and ventricular arrhythmias; 2 developed low platelet counts. The trial was stopped early and the drug is not likely to move forward in its current formulation.
A number of surprising studies showed that predictions as to how viruses might adapt to attachment inhibitors may be wrong. For instance, Xu and colleagues (Abstract 398-T) presented evidence that laboratory viruses that evolve high-level resistance to SCH-C in cell cultures are not associated with a switch to CXCR4, but rather simply attach more firmly to CCR5. The possible results of this kind of adaptation on disease course are completely unknown. In another poster, Van Rij and colleagues from the Netherlands (Abstract 394-T) showed that CXCR-4-using "SI" variants that occurred naturally in a patient with AIDS were actually less sensitive than earlier viruses from the same patient that could use both CXCR-4 and CCR5. This may suggest that even CXCR-4 inhibitors could lead to an "X4 switch"-a somewhat scary possibility if these were to be used in patients with early infection. All in all, the 1st generation of attachment inhibitors have not fared very well in their early trials, and the future of both CCR-5 and CXCR-4 inhibition remain clouded by doubt.
OTHER CANDIDATE ENTRY INHIBITORS
The "holy grail" of entry inhibition is the search for small molecule entry inhibitors that may be given orally. [Abstract 309 gave an update on this search at NIH]. For now, clinicians are trying to figure out ways that injectable fusion and entry inhibitors may be used-for instance as part of "deep salvage" therapy for treatment-experienced patients, "induction" therapy for treatment naive patients, or in eradication protocols. If "cellular resistance" proves to be important as some people think (with cells actively pumping drug out of the interior), then drugs working on the outside of the cell may ultimately be critical tools in our armamentarium. Interest in attachment and fusion inhibitors appears to be here to stay.
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