Debt obligations consisted of the following as of December 31, 2019 and 2018 (in thousands):

Debt instrument	Principal at December 31, 2019		Unamortized Debt Discount			Balance at December 31, 2019
Current Portion:
Debentures	$	146	$	—		$	146
2015 Subordinated Notes		500		—			500
Long-term Portion:
2015 Subordinated Notes		13,500		(120	)		13,380
Total	$	14,146	$	(120	)	$	14,026

Debt instrument	Principal at December 31, 2018		Unamortized Debt Discount			Balance at December 31, 2018
Current Portion:
Debentures	$	146	$	—		$	146
Long-term Portion:
2015 Subordinated Notes		14,000		(788	)		13,212
Total	$	14,146	$	(788	)	$	13,358

On February 20, 2015, we, certain existing investors and certain additional investors entered into an Amended and Restated Note Purchase Agreement, pursuant to which we (i) canceled our senior subordinated promissory notes issued in April 2013 (the “2013 Notes”) in exchange for new senior subordinated promissory notes (the “2015 Subordinated Notes”) in the aggregate principal amount of $5.0 million, (ii) issued additional 2015 Subordinated Notes in the aggregate principal amount of $9.0 million and (iii) issued five year warrants (the “2013 Warrants”) to purchase 1,400,000 shares of our common stock at an exercise price of $5.10 per share.

The 2015 Subordinated Notes bear interest at a rate of 8% per annum, payable in cash on the first day of each month in arrears. Among other default and acceleration terms customary for indebtedness of this type, the 2015 Subordinated Notes include default provisions which allow for the noteholders to accelerate the principal payment of the 2015 Subordinated Notes in the event we become involved in certain bankruptcy proceedings, become insolvent, fail to make a payment of principal or (after a grace period) interest on the 2015 Subordinated Notes, default on other indebtedness with an aggregate principal balance of $13.5 million or more if such default has the effect of accelerating the maturity of such indebtedness, or become subject to a legal judgment or similar order for the payment of money in an amount greater than $13.5 million if such amount will not be covered by third-party insurance. The 2015 Subordinated Notes are not convertible into shares of our common stock and were to mature on February 20, 2020, at which point we would have been required to repay the full outstanding balance in cash. In February 2020, we amended $13.5 million of the 2015 Subordinated Notes, extending the due date by three years to February 2023, the remaining $0.5 million of the 2015 Subordinated Notes were repaid in February 2020. As a result, the obligation has been classified as non-current in the accompanying consolidated balance sheet. Refer to Note 23, Subsequent Events for further detail. The Company may prepay the 2015 Subordinated Notes at any time, in part or in full, without premium or penalty.

The warrants to purchase 500,000 shares of the Company’s common stock issued in connection with the 2013 Notes (the “2013 Warrants”) had an exercise price of $4.41 per share; and expired on April 15, 2019.

In March 2017, we and the holders of the 2015 Subordinated Notes entered into an Amendment to Notes and Warrants, pursuant to which we (i) extended the term of the 2013 Warrants by two years from April 15, 2017 to April 15, 2019 and (ii) extended the maturity date of the 2015 Notes by two years from February 20, 2018 to February 20, 2020. This resulted in an additional debt discount of $0.7 million, which will be amortized using the effective interest method over three years, the expected life of the 2015 Subordinated Notes. The 2013 Warrants and 2015 Notes were otherwise unchanged. The Amendment to Notes and Warrants was accounted for as a debt modification.

In January 2018, we through our wholly-owned subsidiary, Antigenics, entered into a Royalty Purchase Agreement (the “HCR Royalty Purchase Agreement”) with Healthcare Royalty Partners III, L.P., and certain of its affiliates (collectively “HCR”), and we used $161.9 million of the upfront proceeds from HCR to redeem all of our limited recourse notes (the “Notes”) dated September 8, 2015, accordingly, the related note purchase agreement and the Notes issued thereunder were redeemed in full and terminated. In connection with this redemption, we recorded a $10.8 million loss on early extinguishment of debt which primarily reflects the payment of premiums to fully redeem the notes and the write-off of unamortized debt issuance costs and discounts. See Note 18 for additional information on the Royalty Purchase Agreement.

The Notes accrued interest at a rate of 13.5% per annum, compounded quarterly, computed on the basis of a 360-day year and the actual number of days elapsed. The Notes had limited recourse and were secured solely by a first priority security interest in the royalties and accounts and payment intangibles relating thereto plus various rights of Antigenics related to the royalties under its contracts with GSK.

The redemption price was equal to the outstanding principal amount of the Notes, plus all accrued and unpaid interest thereon, plus a premium payment that would yield an aggregate internal rate of return (“IRR”) for the purchasers of 17.5% in accordance with the terms of the NPA.

No non-cash interest expense related to the Notes was recorded for the year ended December 31, 2019. We recorded $849,000 and $17.4 million in non-cash interest expense related to the Notes for the years ended, December 31, 2018 and 2017, respectively, within our consolidated statement of operations and comprehensive loss.

At December 31, 2019, approximately $146,000 of debentures we assumed in our merger with Aquila Biopharmaceuticals are outstanding. These debentures carry interest at 7% and are callable by the holders. Accordingly, they are classified as short-term debt.