Investors have acted with alarm to the news that Eli Lilly and Daiichi Sankyo have called a halt to enrolment for two mid-stage studies of their much-touted antithrombotic prasugrel, a potential rival to the blockbuster Plavix.

Lilly and its Japanese partner said that they have suspended enrolment in two “small” Phase II trials of prasugrel as a result of pharmacokinetic analyses which indicate that “a dose adjustment may be appropriate for certain subpopulations”.

The two pharmacodynamic studies compare the levels of inhibition of platelet aggregation in patients with coronary artery disease taking prasugrel or Sanofi-Aventis/Bristol-Myers Squibb’s blockbuster Plavix clopidogrel. Neither study has an efficacy endpoint, Lilly said, noting that patient enrolment will resume “as soon as additional analyses of pharmacokinetic and clinical data are completed, and the protocols are amended and approved by institutional review boards”.

Anthony Ware, Lilly’s cardiovascular platform leader for prasugrel, insisted that “these amendments are strictly protocol-related and do not provide a basis for inferring overall outcomes of other prasugrel trials". Regardless of these soothing comments, however, fears are growing about the drug.

Fears over Phase III data

Steve Scala, an analyst at Cowen & Co, has downgraded Lilly to ‘neutral,’ while reducing his estimates for the firm, claiming that that upcoming Phase III data from the TRITON TIMI-38 trial is expected to indicate that prasugrel’s bleeding rate is unfavourable when compared to Plavix. Nevertheless, the former drug should exhibit greater effectiveness and prove to be approvable, though the upcoming data is expected to exert pressure on Lilly’s share price.

Plavix is expected to come off-patent in the USA in 2011 and if prasugrel is found to be more effective, it could have peak sales of $4 billion and more if Lilly and Daiichi manage to get the drug onto the market before generic clopidogrel is introduced.