In February, Merck KGaA delivered a new vote of confidence
for hypoxia-inducible prodrugs. On the heels of phase 2 trial
success in pancreatic cancer, the Darmstadt, Germany–based
pharma signed a deal with Threshold Pharmaceuticals of S.
San Francisco, California, worth $25 million upfront, a total
of $525 million in milestones and double-digit royalties to
jointly develop and commercialize TH-302, a 2-nitroimidazoletriggered
bromo analog of the approved DNA-alkylating agent
ifosfamide. Pancreatic cancers have a particularly high rate of
hypoxia and >90% of tumors bear hypoxic regions. “My guess
is that served as the basis for Threshold’s deal with Merck,”
says John Gutheil, CEO of ProActa, a La Jolla, California–based
biotech that is currently pursuing another prodrug PR-104,
a phosphate precursor of the dinitrobenzamide mustard for
treating acute leukemia.
Threshold’s phase 2 results, released in March at the
American Association for Cancer Research in Chicago, signal
a revival of sorts for the prodrug paradigm. In a 214-patient
randomized, controlled trial called TC-CR-404, TH-302 in
combination with Gemzar (gemcitabine) showed an advantage
in median progression-free survival of 5.6 months compared
with 3.6 months in patients on Gemzar alone. The response
rate was also higher on the prodrug combination: 22%
compared with 12% on Gemzar alone.
The same month, Threshold also received an orphan
drug designation for TH-302 from both European Union
(EU; Brussels) and US regulators for soft-tissue sarcoma.
Currently in a pivotal phase 3 trial with the Sarcoma Alliance