Synta Pharmaceuticals Corp suspended a late stage trial of its experimental drug for melanoma after safety concerns were raised by an independent data monitoring committee, including an increased number of deaths in patients on the drug.

Touted as one of the upcoming small biotech companies, Synta shares plummeted by 73 % to $1.75 in after-hours trade and closed at $6.39 Thursday on Nasdaq. The company was developing the drug elesclomol, with British drug maker, GlaxoSmithKline, the world's second largest drug maker.

Simos Simeonidis, a biotechnology analyst at the firm Rodman & Renshaw said, "This potentially kills this class of drugs. It will be tough for them to study it in other tumors." Elesclomol was to work on Synta's theory that cancer cells were more sensitive to oxidative stress than healthy cells and by increasing oxidative stress, the cancer cells would self destruct.

Called SYMMETRY, the late stage trial was doing a comparison on the drug elesclomol in combination with the chemotherapy drug paclitaxel with standalone paclitaxel in chemo-naive patients with metastatic melanoma.

The drug elesclomol had seemed to show a slowing of tumor progression but the company stopped the study as excess deaths were seen in the drug group, although what caused these deaths was not clear.

Safi Bahcall, Synta's chief executive calling the results "stunningly disappointing" said, "There is nothing short of a train falling on my head that could have surprised me more."

Synta said additional studies with elesclomol which included one in hormone-refractory metastatic prostate cancer and a monotherapy dose escalation study would also be suspended till the SYMMETRY trial went through additional analysis.

Synta said it would discuss about the future development of elesclomol with Glaxo. GlaxoSmithKline had already paid $130 million in milestone and upfront payments to Synta and had pledged $1 billion for the drug if it worked.