Enhanced Trofile assay better at identifying HIV tropisms, only slight impact on virologic outcomes

A new test is better at pinpointing who might benefit most from CCR5 antagonists for HIV treatment, but when two major studies of CCR5 antagonists were re-analysed, excluding data from people who were screened out by the new tropism assay, it made only a modest difference to the proportions who had a good viral load response, according to two studies presented this week at the 48^th Interscience Conference on Antimicrobial Agents and Chemotherapy in Washington DC.

Tropism re-analyses

Maraviroc and vicriviroc are only effective against R5-tropic strains of HIV (i.e., strains which enter CD4 cells by using the CCR5 co-receptor on the cell surface). People with X4-tropic strains (which use the CXCR4 co-receptor exclusively), with dual-tropic strains (which use both), or who have a mixture of these viral strains, do not benefit as well from CCR5 inhibitors.

Before patients are started on either of the new CCR5 antagonists maraviroc (Celsentri) or vicriviroc, they are screened to identify whether they have the R5-tropic strain of HIV that responds to these drugs. This screening is accomplished with Monogram Biosciences' Trofile assay, the original version of which has now been phased out and replaced in clinical practice by an enhanced, more sensitive version (Trofile-ES).

Studies to date have screened patients with a previous, less sensitive version of the tropism assay developed by Monogram Sciences, Trofile. This assay has now been superceded by a version with enhanced sensitivity - Trofile-ES.

Prospective analyses of the impact of this new assay have not yet been done. In two presentations at the 48^th Interscience Conference on Antimicrobial Agents and Chemotherapy, investigators reanalysed patient data from existing trials to examine the difference between the old and new assay.

The first of these was the ACTG5211 study, which evaluated different doses of vicriviroc versus placebo (in addition to optimised background therapy, or OBT) in treatment-experienced people. Ongoing findings from ACTG5211 have included 48-week data presented at the Fourth IAS Conference in Sydney in 2007. At week 48 in this study, participants receiving vicriviroc had median drops in viral load of between -1.44 and -1.92 log₁₀, and 27% to 37% achieved an undetectable viral load, compared with 11% in the placebo arm.

The other was the MERIT study, which evaluated maraviroc against efavirenz, in combination with AZT/3TC (Combivir). While other studies had examined maraviroc in treatment-experienced patients, the MERIT trial was conducted in people who had not taken antiretrovirals before. Forty-eight week MERIT findings, also presented in Sydney, showed that maraviroc was slightly less virologically effective than efavirenz but resulted in fewer side-effects.

In these studies, the investigating teams re-analysed the data from ACTG5211 and MERIT, using the enhanced Trofile-ES on stored blood plasma samples. The object was to assess whether the enhanced assay could more accurately identify patients in whom vicriviroc and maraviroc would have the most benefit.

The results showed that Trofile-ES was considerably better at identifying patients with very low levels of dual/mixed-tropic (DM) virus which the earlier assay missed. (Purely X4-tropic virus is rare, and no participants in either study had it at study entry).

This "tighter" screening narrowed the pool of "suitable", R5-tropic patients by 15 to 22%. Compared to the larger treated groups in the full studies, this narrower pool did show better virologic responses, although the differences were relatively small.

Vicriviroc: ACTG5211

In the original ACTG5211 study, 118 R5-tropic patients were identified and enrolled, of whom 90 were randomised to receive vicriviroc and 28 to placebo, plus optimised background therapy (OBT).

When Trofile-ES was used on stored patient samples, 89 and 25 subjects were found to have had R5-tropic (R5) and dual/mixed-tropic (DM) virus, respectively, at screening; samples from 4 subjects were not available. In other words, Trofile-ES was able to identify a further 25/114 (22%) of patients who would not be considered eligible due to having DM virus.

In validation tests, Trofile-ES could detect DM virus with high accuracy even when it made up roughly 0.1% of the total viral sample (about tenfold lower than the standard Trofile assay).

As expected, patients with only R5-tropic virus according to the enhanced assay had higher reductions in viral load than those with DM virus.

The next question was whether the more restrictive screening resulting from the enhanced assay would result in better virological outcomes than the original assay. While slight trends toward better outcomes were seen, the differences were small and not statistically significant. At day 14 and at week 24, the drops from baseline viral load were roughly 0.1 to 0.2 log greater in the group identified by the enhanced assay. At week 24, slightly higher proportions of people in the enhanced-screening group achieved viral load drops > 1 log, viral load levels below 400 copies/ml, and viral load levels below 50 copies/ml.

Maraviroc: the MERIT study

As in the 5211 reanalysis, screening samples from MERIT patients enrolled on the basis of an R5 result according to the original Trofile were retested with Trofile-ES. Across both treatment arms, an additional 106 of 721 patients (15%) were excluded for dual/mixed tropism using the new test.

Compared to the original MERIT results, maraviroc showed better results with the Trofile-ES screening, and these results were more comparable to those with efavirenz. In the original MERIT study at 48 weeks, by ITT analysis, 65% of participants on maraviroc/Combivir had viral loads below 50 copies/ml, compared to 69% on efavirenz/Combivir – just short of the non-inferiority threshold for the study. However, in the patients screened by Trofile-ES, 68% in both arms achieved viral suppression. There were also fewer discontinuations in the re-screened maraviroc arm, both overall and specifically due to lack of efficacy.

Conclusions

In summary, the enhanced tropism assay Trofile-ES proved significantly better at identifying dual/mixed-tropic HIV than the standard assay. The ACTG5211 investigating concluded that "Trofile-ES had improved ability to predict antiretroviral response to vicriviroc" in treatment-experienced patients, and similar results were seen in the MERIT study of maraviroc in treatment-naïve individuals. However, the improvement in virologic responses was quite modest.

The implications of a more sensitive, and therefore more restrictive, assay were briefly discussed in the question period after Dr. Su's presentation. As increasingly sensitive assays can only serve to screen more prospective patients away from CCR5-antagonist treatment, some delegates questioned whether this might deny some residual benefit to the excluded patients. It is important to recognise that the newer assay does not, in fact, "improve" outcomes for any individual patients. It merely better selects those who receive the most benefit.

However, since the additional advantage seen in these reanalyses was marginal, this implies that the patients who might now be excluded by enhanced screening (i.e., those with very small populations of dual/mixed-tropic virus) might actually enjoy outcomes nearly as good as those of patients who completed the trials. i.e., (as one delegate phrased it) by "slavishly excluding people based on assay outcome", we could be withholding potentially beneficial (even if not fully optimal) treatment from these patients.

The probable implication is that tropism test results should be interpreted and acted upon with informed, expert judgment – as is currently the case with resistance testing for other antiretroviral agents.