The FDA issued an approvable letter for iPlex, a growth hormone deficiency drug from Insmed Inc., less than a month after the product's chief competitor, Tercica Inc.'s Increlex, gained the agency's blessing for the same indication.

Formerly known as SomatoKine, iPlex (rhIGF-1/rhIGFBP-3) (mecasermin rinfibate) is indicated for the treatment of children with growth failure who suffer from severe primary insulin-like growth factor-1 (IGF-1) deficiency. On the positive side, the approvable letter does not ask for any additional clinical studies, and requests only for clarifications that are "pretty minor in nature," regarding chemistry, manufacturing and controls sections of the application, Insmed's President and CEO Geoffrey Allan said during a conference call.

Shares of Insmed (NASDAQ:INSM) jumped 31.2 percent Wednesday to close at $1.43, up 34 cents.

Allan added that the company expects to submit responses within "a short period of time," and, at this point, still plans to launch the product in early 2006.

However, it's uncertain to what effect Increlex's approval might have on the potential marketability of iPlex.

Brisbane, Calif.-based Tercica gained approval late last month for Increlex, its long-term treatment of growth failure in children with severe primary IGF-1. Increlex (rhIGF-1, mecasermin [rDNA origin] injection) was designated an orphan drug for that indication, a status that would guarantee seven years of marketing exclusivity in the U.S.

Last month, Richmond, Va.-based Insmed failed in its attempt to thwart Increlex's approval with a citizen petition asking the FDA to deny Tercica's application. Prior to that, Tercica and its licensor, South San Francisco-based Genentech Inc., filed for a preliminary injunction to prevent Insmed from making and selling its drug. Tercica later withdrew the motion, but a patent infringement lawsuit alleging that Insmed infringed on three patents relating to the combination of IGF-1 and IGFBP-3, and its treatment for primary IGF-1 deficiency has not yet been resolved. (See BioWorld Today, Aug. 19, 2005.)

Though iPlex also received orphan drug designation, Increlex was able to beat it to the finish line after the FDA delayed by three months the review date for Insmed's drug. At this time, Allan reported that the FDA has not yet addressed "some orphan exclusivity issues," but said the company "cannot comment further."

Both products are similarly designed, though iPlex is given as a once-daily injection while Increlex is administered two times per day.

An estimated 6,000 children in the U.S. suffer from primary IGF deficiency, a condition characterized by abnormally low blood IGF-1 levels in the presence of normal or elevated endogenous growth hormone. Adequate levels of IGF-1 are needed for the normal growth of bones, cartilage and organs in children.

IPlex, a recombinant protein complex of IGF-1 and its binding protein, insulin-like growth factor binding protein-3 (IGFBP-3), has demonstrated statistical significance in growth rate and height velocity, said Kenneth Attie, chief medical officer.

Results of the company's pivotal study, presented last week at the European Society Pediatric Endocrinology/ Lawson Wilkins Pediatric Endocrine Society 7th Joint Meeting in Lyon, France, showed a "continued efficacy in terms of growth response, and good tolerance and safety data," he said.

Efficacy data from 25 patients showed that height velocity correlated with dosage levels and IGF blood levels. Of the children dosed with up to 1 mg/kg of iPlex, those who had IGF levels in the target range grew about 8.2 cm in the first year, while the patients who had below normal levels grew 5.6 cm per year.

"Overall, even the low-dose group had high statistical significance in height velocity," Attie said.

A subset of patients, who were dosed up to 2 mg/kg, grew an average of 8.2 cm per year, according to nine-months of data. That subset, who suffered the most severe deficiency, had grown an average of only 2.2 cm per year prior to receiving treatment, Attie added.

"We also continued to find that the once-daily administration is safe and well tolerated," he said, and even though antibodies to the protein were observed during treatment, extensive testing showed that they did not neutralize the drug's biological activity.

In preparation of final FDA approval, Insmed said it is putting together a sales force to market iPlex. Chief Business Officer Philip Young said the company will have sales people in place "as soon as we have a visual as far as the time" for approval and launch.

In addition to primary IGF-1 deficiency, Insmed also is evaluating iPlex as a treatment for diabetes, severe burn injuries and hip fractures.

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