NOVATO -- Raptor Pharmaceutical Corp. (Nasdaq: RPTP) on Thursday reported it narrowed its fourth-quarter loss as sales pick up for its first approved drug, a treatment of a rare genetic disease that attacks largely the eyes and kidneys.

The period, ended Dec. 31, is the second full quarter to include sales for Raptor's flagship drug, Procysbi, which treats the rare condition nephropathic cystinosis. The U.S. Food & Drug Administration approved it last April, and it became commercially available in the U.S. in June.

Raptor reported a net loss of $12.1 million, or 20 cents per share, for the quarter, improved from a loss of $19.4 million during a four-month transition period a year before. Net loss for the year was $69.4 million, or $1.20 per share.

"The year ended December 31, 2013 was a period of significant achievement for Raptor and I am pleased by our commercial progress and the strong market acceptance for Procysbi," said Christopher Starr, PhD, chief executive officer, in a statement. "We are preparing to discuss with regulators the encouraging 18-month treatment results from our Phase 2/3 clinical trial with RP103 in Huntington's disease in order to determine our next steps in the development of RP103 for this potential indication."

Raptor recognized $10.2 million in quarterly net product sales of Procysbi on $800,000 in cost of sales. Net product sales for the year totaled $16.9 million on cost of sales was $1.7 million.

There were no comparable figures for 2012, because sales began in June. Raptor also shifted fiscal year end to December from August in 2012, and Raptor provided only results for the four-month transition period, rather than just the third quarter.

Research and development expenses for the fourth quarter of 2013 were $7.8 million and $29.2 million for the year 2013. The increase in the current year-over-prior periods was mostly due to higher external costs for clinical studies, lab services and higher staffing expenses, according to Raptor.

Selling, general and administrative expenses increased to $12.4 million for the fourth quarter and were $37.9 million for the year. The increase in the current year over prior periods was primarily due to additional sales and marketing costs for the commercialization of Procysbi in the U.S.

Interest expense for the fourth quarter was $2.7 million and $6.8 million for the year. The increase in interest expense in the current year over the prior periods was attributed to the $50.0 million debt facility the company entered into with HealthCare Royalty Partners in December 2012.

Cash and cash equivalents as of December 31, 2013 were $83.1 million and included approximately $23.7 million of net proceeds from the second tranche of financing under Raptor's loan agreement with HC Royalty Partners, $38.4 million in net proceeds under its at-the-market common stock sales agreement, $10.3 million proceeds from warrant exercises, and $2.5 million proceeds from stock option exercises, all received during the year ended December 31, 2013.

As of Dec. 31, there were 236 new prescriptions for Procysbi, 165 unique patients on the therapy and another 71 patients in the process of reimbursement verification.

The share price for Raptor dropped by more than 4 percent Thursday, closing at $14.79. Following the financial report after the market closed, the price fell by nearly 11 percent to $13.20 in after-hours trading.