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Cell Therapeutics' Upcoming FDA Worries

3. How reliable are the response rates in the EXTEND study?

Before I get to this issue, let's review the details and data (per Cell Therapeutics) that came out of the EXTEND study:

The PIX 301 "EXTEND" trial (download the posters here) enrolled 140 patients with relapsed or refractory aggressive non-Hodgkin's lymphoma (NHL). The patients, all of whom had received two or more prior therapies, were randomized to receive either pixantrone monotherapy or a doctor's choice of other single-agent chemotherapies.

Cell Therapeutics says the EXTEND study met its primary endpoint, with 25.7% of pixantrone patients achieving a complete response (CR) or an unconfirmed complete response (CRu) compared to 7.1% of patients in the control arm achieving a CR/CRu. The result was statistically significant.

With that as background, watch for how the FDA adjudicates the patients deemed to be unconfirmed complete responders (CRu) because in recent years, at least one influential academic group of lymphoma researchers no longer uses CRu as a valid measurement of disease response. Patients are now characterized as complete responders (CR) or partial responders (PR.)

Six patients treated with pixantrone were deemed unconfirmed complete responders, according to Cell Therapeutics. If some or all of these patients are actually found to be partial responders by the FDA, the 25.7% CR/CRu response rate in the study could fall dramatically. If that happens, the EXTEND study may no longer meet its primary endpoint with statistical significance.

The secondary efficacy endpoints of the EXTEND study were also met, according to Cell Therapeutics. Median progression-free survival for pixantrone patients was 5.6 months compared to median PFS of 2.6 months for patients in the comparator arm -- statistically significant.

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