Synta Pharmaceuticals

(SNTA)

was forced to halt a phase III study of its skin cancer drug Thursday after independent safety monitors observed that more patients taking the drug were dying than those in the study's control arm.

Two other cancer trials involving Synta's drug, known as elesclomol, have also been stopped, the company said.

Synta shares were plunging $4.39, or 69%, to $2 in after-hours trading.

Synta had recently completed patient enrollment in the melanoma, or skin cancer, study. Patients with advanced melanoma were randomized to either receive elesclomol plus chemotherapy or chemotherapy alone. The trial was designed to determine whether treatment with elesclomol and chemo would prolong survival or slow disease progression in skin cancer patients compared to treatment with chemo alone.

Results from this study were expected later this year, but Thursday, Synta said that an independent data monitoring committee halted the study prematurely after it was determined that a greater number of patients in the elesclomol arm of the study were dying compared to the control arm.

Synta's partner for elescomol is

GlaxoSmithKline

(GSK) - Get Report

.

Melanoma is one of the most difficult to treat cancers and few drugs in development have proven successful. In recent years, drugs from

Medarex

(MEDX)

,

Bristol Myers Squibb

(BMY) - Get Report

and

Pfizer

(PFE) - Get Report

have all failed phase III studies.

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