General and administrative expenses were $2.9 million for the second quarter of 2013 as compared to $2.3 million for the same period in 2012. The increase was primarily due to increased expenses for personnel, legal services and professional services.
Operating expenses for the six months ended June 30, 2013 were $11.3 million, as compared to $14.9 million for the same period in 2012. Research and development expenses were $5.8 million for the six months ended June 30, 2013 as compared to $9.7 million for the same period in 2012. General and administrative expenses were $5.5 million for the six months ended June 30, 2013 as compared to $5.1 million for the same period in 2012.
Other expense was $588,000 for the second quarter of 2013 as compared to $460,000 for the same period in 2012. The increase in other expense is primarily the result of $958,000 in interest expense and amortization of debt issuance costs related to the debt transaction between BioPharma II and Curis Royalty, a wholly-owned subsidiary of the Company. Interest expense was partially offset by $333,000 in other income recorded as a result of a decrease in the fair value of a warrant liability during the second quarter of 2013. Other expense during the prior year period primarily represents the change in the fair value of this warrant liability. Other expense was $1.2 million for the six months ended June 30, 2013 as compared to $434,000 for the same period in 2012.
As of June 30, 2013, Curis' cash, cash equivalents, marketable securities and investments totaled $57.1 million and there were approximately 81.7 million shares of common stock outstanding.
In December 2012, Curis Royalty entered into a debt transaction with BioPharma II pursuant to which BioPharma II loaned Curis Royalty $30 million at an annual interest rate of 12.25%. The loan is secured with the Erivedge royalty and royalty-related payments that arise under the collaboration agreement with Genentech, all of which payment rights were transferred and assigned to Curis Royalty by Curis at the time of the loan transaction. In addition, pursuant to the terms of the loan, certain of the royalty and royalty-related payments received by Curis Royalty are used to pay down the loan, subject to specified caps. For 2013, Curis Royalty is required to pay BioPharma II up to $1 million per quarter of the royalty revenues that it receives from Genentech/Roche. The loan constitutes an obligation of Curis Royalty and is intended to be non-recourse to the Company.