Loan Payable

As of December 31, 2018 and 2017, long-term debt consisted of the following (in thousands):



	December 31,
	2018			2017
Term Loans
Term Loan A - 6.9% principal	$	7,619		$	10,000
Term Loan B - 6.9% principal	7,619			10,000
Term Loan C - 7.4% principal	12,000			15,000
Term Loan D - 8.8% principal	663			—
Term Loan E - 8.8% principal	7,098			—
Final fee obligation	3,288			2,429
Repayment of principal	—			(7,762		)
Unamortized issuance costs	(2,245		)	(1,641		)
Total debt, net	36,042			28,026
Current portion of long-term debt	—			(7,927		)
Long-term debt	$	36,042		$	20,099

Term Loans

In January 2015, the Company entered into a Loan and Security Agreement, or the LSA, with Solar Capital Partners (as successor-in-interest to General Electric Capital Corporation), and certain other financial institutions party thereto, as lenders, pursuant to which the Company obtained (a) up to $35,000,000 in a series of term loans and (b) a revolving loan in the maximum amount of $5,000,000. The term loans will accrue interest at a rate equal to (a) the greater of 1.00% or the 3-year treasury rate in effect at the time of funding, plus (b) an applicable margin between 4.95% and 5.90% per annum.

In September 2018, the Company amended its LSA to extend the interest-only period for amounts borrowed by the Company under the Agreement until January 1, 2020. The Company’s payments are due based upon a 24-month amortization schedule with the remaining outstanding principal and accrued interest amounts due on January 1, 2021, or the Final Maturity Date. The Company obtained additional term loans of $663,000 and $7,098,000, respectively, during the year ended December 31, 2018.

As of December 31, 2018, the Company had borrowed all $42,762,000 under the term loans as provided in the LSA, and the Company had not borrowed any of the $5,000,000 available under the revolving loan. The Company will be required to pay a $20,000 annual management fee due on each anniversary of the closing date of the Amendment and a $3,288,000 final payment fee due on the Final Maturity Date. Interest expense recognized on the term loans for the years ended December 31, 2018, 2017, and 2016 totaled $2,018,000, $1,820,000, and $1,184,000, respectively.

Under the LSA, the Company is required to comply with certain affirmative and negative covenants, including, without limitation, delivering reports and notices relating to the Company’s financial condition and certain regulatory events and intellectual property matters, as well as limiting the creation of liens, the incurrence of indebtedness, and the making of certain investments, dividends, payments and acquisitions, other than as specifically permitted by the LSA. The LSA includes customary events of default, including instances of a material adverse change in our operations, that could require immediate payment of the Company's obligations. As of December 31, 2018, the Company was in compliance with all covenants under the LSA.

Revolving Loan

Pursuant to the LSA, the Company may borrow up to $5,000,000 under the revolving loan facility. Borrowings under the revolving loan will accrue interest at a rate equal to (a) the greater of 1.25% per annum or a base rate as determined by a three-month LIBOR-based formula, plus (b) an applicable margin between 2.95% and 3.95% based on certain criteria as set forth in the LSA. All principal and interest outstanding under the revolving loan is due and payable on the Maturity Date. Following the funding of Term Loan A, the Company is required to pay a commitment fee equal to 0.75% per annum of the amounts made available but unborrowed under the revolving loan. As of December 31, 2018, the Company had not borrowed any amounts pursuant the revolving loan facility. Interest expense recognized for the unused revolving loan facility fee for the years ended December 31, 2018, 2017, and 2016 was $35,000, $35,000, and $42,000, respectively.

Debt Issuance Costs

As of December 31, 2018 and 2017, the Company had $2,245,000 and $1,641,000, respectively, of unamortized debt issuance discount, which is offset against borrowings in long-term and short-term debt.

For the twelve months ended December 31, 2018, 2017, and 2016, amortization of debt issuance costs were $938,000, $1,132,000, and $298,000, respectively, which was included in interest expense in the Company's consolidated statements of comprehensive loss for the periods presented.

Letter of Credit

In September 2012, the Company provided a $758,000 letter of credit issued by Banc of California to the landlord of its executive office facility in Carlsbad, California. This letter of credit was secured with $758,000 of restricted cash as of December 31, 2018.