Through March 31, 2019, the Company borrowed $15.0 million under the Loan and Security Agreement (the “Loan Agreement”) by and among the Company, certain subsidiaries of the Company from time to time party thereto, the several banks and other financial institutions or entities from time to time parties thereto (collectively referred to as “Lender”) and Hercules Capital, Inc. (“Hercules”), in its capacity as administrative agent and collateral agent for itself and Lender, and incurred $0.4 million of related debt discount and issuance costs, inclusive of the $0.3 million fee paid upon closing. The debt discount and issuance costs are being accreted to the principal amount of debt and being amortized from the date of issuance through August 1, 2021 to interest expense using the effective-interest method. In addition, the Company accrues the related 5.55% final fee payment of $0.8 million due at maturity to outstanding debt by charges to interest expense using the effective-interest method over the same period. The effective interest rate of the outstanding debt under the Loan Agreement is approximately 12.7%.

(in thousands)	March 31, 2019
Note payable	$	15,000
Less: current portion		(1,475	)
Note payable, net of current portion		13,525
Debt discount, net of accretion		(255	)
Accretion related to final payment		249
Note payable, net of discount and current portion	$	13,519

During the three months ended March 31, 2019, the Company recognized $0.5 million of interest expense related to the Loan Agreement. No interest expense was associated with the Loan Agreement for the three months ended March 31, 2018.

Estimated future principal payments due under the Loan Agreement are as follows as of March 31, 2019:

(in thousands)	Note Principal Payments
Remainder of 2019	$	—
2020		8,394
2021		6,606
Total minimum note principal payments	$	15,000