Talon: Assessing Marqibo's FDA Odds

SAN MATEO, Calif. ( TheStreet) -- An FDA advisory panel convenes March 21 to review Marqibo, a leukemia drug from Talon Therapeutics ( TLON). A breakdown of what to expect:

What are Marqibo's chances for a positive panel vote?

Small. I won't guarantee a negative vote; there's always a chance the panel could look favorably on the drug, but the odds are against.

But you're a biotech basher. You hate everything.

It's not just me. The market isn't exactly pricing in good news for Talon -- a bulletin board stock with a $20 million-plus market value.

By the way, I don't hate everything!

I was under the impression that the Marqibo data were strong and that the drug helped leukemia patients who had run out of other treatment options. Isn't this an argument in favor of approving the drug?

It's probably THE argument Talon and its advisors will make during next week's advisory panel meeting. A majority of the 65 acute lymphoblastic leukemia (ALL) patients, all adults over 18, enrolled in the Marqibo study were on their third or fourth lines of therapy. A smaller group of patients in the study had cancer that was no longer responding to five or six previous treatments.

Wait, you just said something vaguely positive about Talon. Keep going.

Thirteen, or 20%, of the 65 ALL patients achieved a complete or near-complete tumor response -- the primary endpoint of the study. The median duration of response for those patients with a CR/CRi was 5.4 months with a median overall survival of 7.4 months.

When stratified by previous therapies, those ALL patients no longer responding to three prior therapies demonstrated a 17% CR/CRi rate from Marqibo therapy; fourth-line ALL patients had a 24% CR/CRi rate to Marqibo.

And these results aren't persuasive enough for an FDA panel to recommend approval?

Meh. Talon's study was single arm, meaning all the patients received treatment with Marqibo. It's difficult to assess patient benefit when there is nothing to compare against Marqibo therapy.

FDA has approved cancer drugs based on single-arm studies in the past, but the agency has recently become a lot more nit-picky about it (and that's doubly true for single-arm registration trials for blood-related cancers.)

No approval standard exists for single-arm cancer trials but a good rule of thumb is a 25% response rate with a six-month duration of response. Marqibo falls short on both measures.

You sound like the old biotech-bashing Adam again.

Hold on! It's not just me telling you something is likely amiss with the Marqibo review, take a look at the warning Talon discloses in its SEC filings:

"In connection with submitting our NDA, we also requested that the FDA grant our application 'priority review,' which would reduce the FDA's review period from the standard 10 months to six months. Although our NDA was accepted for filing under Subpart H accelerated approval regulations, the FDA, upon preliminary review, did not grant our request for priority review, as indicated by the PDUFA date of May 13, 2012. We believe the FDA's determination that our NDA did not qualify for priority review may reduce the probability that the agency will ultimately grant accelerated approval of Marqibo."

That's not encouraging. What does it mean?

By disqualifying Marqibo for priority (six-month) review, FDA is saying the drug does not meet an unmet medical need -- unusual for a cancer drug, for sure. We'll get a fuller explanation of the FDA's criticisms on Monday, March 19 when the agency's briefing documents for the Marqibo panel are posted online.

If I had to guess, I'd say FDA has issues with Talon's trial design and/or the type of ALL patients screened into the study. Remember, too, Marqibo is just a reformulation of an older chemotherapy agent, vincristine.

What? Marqibo is a retread?

Sorta. Talon makes Marqibo by surrounding vincristine in a liposome (fat) bubbles. The bubbles are supposed to allow higher doses of vincristine to be delivered to tumors, thereby increasing efficacy while also reducing side effects?

FDA may have an issue with these claims for lack of data. Talon and FDA have already agreed on a follow-on clinical trial that will compare Marqibo against standard vincristine in treatment-naive ALL patients. The primary endpoint of this study is overall survival, the gold standard for cancer drug clinical trials.

FDA may want to see data from this study before approving Marqibo, hence the lack of priority review.

You haven't mentioned anything about Marqibo's safety?

The 30-day mortality in the Marqibo study was 12%, which works out to eight patients. That may be a cause for concern. Thirty-two percent of patients reported serious febrile neutropenia, or a fever brought on by infections caused by abnormally low white blood cells. The rate of serious neuropathy, or nerve damage/pain, in Marqibo study was 22%, although many patients entered the study with existing neuropathy issues.

But if FDA had already made up its mind to reject Marqibo, why hold an advisory panel next week?

Good question. FDA officials and Talon may be going back-and-forth about the need for new treatment options in ALL, especially for very advanced patients. When the Marqibo data were presented at the American Society of Clinical Oncology annual meeting in June 2010, the point was hammered home that the 20% complete response rate stacked up very well against the historical single-digit response rate observed in similar, advanced ALL patients treated with current single-agent therapies.

Historical precedent also suggests a low regulatory bar at FDA for approval of new ALL drugs. FDA granted approval of Genzyme's (now part of Sanofi ( SNY)) Clolar, based on a similar 20% complete response rate. A major difference between Clolar and Marqibo is that the former was approved for pediatric ALL while Marqibo is seeking approval for patients over 18.

What are the key dates to remember for Talon's Marqibo panel?

First up is Monday, March 19. That's when FDA will release briefing documents for the panel, which will include the agency's review of the Marqibo data. Expect Talon's stock to be volatile based on the findings of the FDA review.

The actual FDA panel is scheduled for Weds. March. 21, from 8 am to 12:45 pm EDT. Expect Talon's stock to be halted for trading on Wednesday, at least until after the panel ends.

--Written by Adam Feuerstein in Boston.

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Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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