Topsalycin in the treatment of prostate cancer: an update

Some 3 years ago we first commented on a Phase II trial of a drug now called topsalycin (but then known as PRX302) in the treatment of low- to intermediate-risk, localized prostate cancer.

Three trials of topsalycin now appear to have been completed in the treatment of prostate cancer, and the results appear to be open to differing interpretations.

The data from the most recent trial were reported in a media release issued yesterday (December 17) by the developing company (Sophiris Bio).

In that trial, the company had experimented to see whether giving patients a second, transperitoneal, intraprostatic injection of topsalycin had greater efficacy than just a single injection. There appears to have been no indication that this was the case.

This, what we know from the completed studies is the following:

A single administration of topsalycin and associated follow-up biopsy data at 6 months post-treatment show that:

  • 15/37 patients (41 percent) exhibited a partial response to treatment, defined as a reduction in maximum cancer core length (MCCL) and/or Gleason pattern, but the targeted lesion was still deemed clinically significant.
  • 12/37 patients (32 percent) did not respond to treatment, defined as no change in the targeted lesion or an increase in MCCL and/or Gleason pattern.

Despite the fact that these data are at best “equivocal”, it appears that Sophiris Bio is willing to press forward with a Phase III clinical trial of topsalycin in the treatment of low- and intermediate-risk prostate cancer, although no details about this trial are available to date, and they will likely have to be based on discussions with regulatory authorities in Europe and in the US.

Since we have no way to know whether a change in Gleason score or any reduction in MCCL on a post-treatment biopsy compared to a diagnostic or standardized pre-treatment biopsy is clinically meaningful, it is a little unclear to your sitemaster what Sophiris is hoping to be able to use as an endpoint in a randomized, Phase III clinical trial.

The idea of being able to give one or more injections of a drug directly into the prostate as a method to treat localized prostate cancer is obviously very attractive. Proving the clinical efficacy of such a treatment may be a good deal more difficult, however.


2 Responses

  1. Sophiris had done a Phase IIa trial in the past which could be compared with the Phase 2b trial data. Refer to Table 4 (patient data that’s ranked by Gleason score) in the patent application for this product. The data was not ranked by MCCL.

    In the interim readout on the Phase IIb trial there were 13 PRs and 10 CRs, and of the 10 CRs six were complete ablations. Eleven responders, those that haven’t reached complete ablation, elected to take a second dose which none of them responded at the trial completion. I assume the four remaining CRs from the first dose elected to continue with a second dose, but that doesn’t really matter.

    In the end the second dose didn’t do anything. It didn’t reduce Gleason scores or MCCL. There’s nothing that shows a secondary injection providing any benefit. What population base would take this drug? I’m scratching my head about why insurance would provide first-line coverage for something that wasn’t going to provide consistent and repeatable results.

  2. Dear Jackson:

    Long before any insurer starts to consider whether they would provide coverage for a product like this, we need to be sure that the FDA will consider approval of a drug like this at all. That isn’t going to happen on the basis of the Phase IIa and Phase IIB data available to date.

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