We lease our facilities in Emeryville, California and Düsseldorf, Germany. We lease and sublease certain manufacturing and office space with lease terms ranging from 3 to 12 years. These leases require monthly lease payments that may be subject to annual increases throughout the lease term. Certain of these leases also include options to renew or extend the lease for two successive five-year terms. These optional periods have not been considered in the determination of the right-of-use assets or lease liabilities associated with these leases as we did not consider the exercise of these options to be reasonably certain.

We also sublease one of our leased premises to a third party. Rent is subject to scheduled annual increases and the subtenant is responsible for certain operating expenses and taxes throughout the life of the sublease. The sublease term expires on March 31, 2031, unless earlier terminated, concurrent with the term of our lease. The subtenant has no option to extend the sublease term. Sublease income for the three and six months ended June 30, 2023 was $2.0 million and $3.6 million, respectively. Sublease income for the three and six months ended June 30, 2022 was $2.0 million and $3.6 million, respectively. Sublease income is included in other income (expense) in our condensed consolidated statements of operations. Rent received from the subtenant in excess of rent paid to the landlord shall be shared by paying the landlord 50% of the excess rent. The excess rent is considered a variable lease payment and the total estimated payments are being recognized as additional rent expense on a straight-line basis.

Our lease expense comprises of the following (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2023		2022		2023		2022
Operating lease expense	$	1,392	$	1,867	$	2,780	$	3,474

Cash paid for amounts included in the measurement of lease liabilities for each of the six months ended June 30, 2023 and 2022 was $3.5 million, and were included in change in lease liabilities in our condensed consolidated statements of cash flows.

The balance sheet classification of our operating lease liabilities was as follows (in thousands):


	June 30, 2023		December 31, 2022
Operating lease liabilities:
Current portion of lease liabilities (included in other current liabilities)	$	4,150	$	3,631
Long-term portion of lease liabilities		31,545		32,801
Total operating lease liabilities	$	35,695	$	36,432

As of June 30, 2023, the maturities of our sublease income and operating lease liabilities were as follows (in thousands):


Years ending December 31,	Sublease Income		Operating Lease Liabilities
2023 (remaining)	$	2,790	$	3,670
2024		5,684		7,431
2025		5,854		6,806
2026		6,030		6,004
2027		6,211		6,044
Thereafter		21,500		21,242
Total	$	48,069		51,197
Less:
Present value adjustment				(15,502	)
Total			$	35,695

The weighted average remaining lease term and the weighted average discount rate used to determine the operating lease liabilities were as follows:


	June 30, 2023			December 31, 2022
Weighted average remaining lease term	7.1 years			7.6 years
Weighted average discount rate		10.1	%		10.1	%

As of June 30, 2023 and December 31, 2022, our material non-cancelable purchase and other commitments for the supply of HEPLISAV-B totaled $38.3 million and $43.4 million, respectively. As of June 30, 2023 and December 31, 2022, we have no non-cancelable purchase and other commitments for the supply of CpG 1018 adjuvant.

As of June 30, 2023, the aggregate principal amount of our convertible senior notes ("Convertible Notes") was $225.5 million, excluding debt discount of $3.4 million (see Note 7).

During 2004, we established a letter of credit with Deutsche Bank as security for our Düsseldorf lease in the amount of €0.2 million (Euros). The letter of credit remained outstanding through June 30, 2023 and was collateralized by a certificate of deposit for €0.2 million, which has been included in restricted cash in the condensed consolidated balance sheets as of June 30, 2023.

In conjunction with our agreement with Symphony Dynamo, Inc. and Symphony Dynamo Holdings LLC (“Holdings”) in November 2009, we agreed to make contingent cash payments to Holdings equal to 50% of the first $50 million from any upfront, pre-commercialization milestone or similar payments received by us from any agreement with any third party with respect to the development and/or commercialization of cancer and hepatitis C therapies originally licensed to Symphony Dynamo, Inc., including our immune-oncology compound, SD-101. In July 2020, we sold assets related to SD-101 to Surefire Medical, Inc. d/b/a TriSalus Life Sciences (“TriSalus”). We paid $2.5 million to Holdings in August 2020. In each of September 2021 and May 2022, we received $1.0 million from TriSalus because it met a pre-commercialization milestone. We recorded the proceeds as gain on sale of assets in our condensed consolidated statements of operations in the third quarter of 2021 and second quarter of 2022. We paid Holdings $0.5 million in each of September 2021 and May 2022. We included the payments in selling, general and administrative expenses in our condensed consolidated statements of operations in the third quarter of 2021 and second quarter of 2022. No liability has been recorded under this agreement as of June 30, 2023.

From time to time, we may be involved in claims, suits, and proceedings arising from the ordinary course of our business, including actions with respect to intellectual property claims, commercial claims, and other matters. Such claims, suits, and proceedings are inherently uncertain and their results cannot be predicted with certainty. Regardless of the outcome, such legal proceedings can have an adverse impact on us because of legal costs, diversion of management resources, and other factors. In addition, it is possible that a resolution of one or more such proceedings could result in substantial damages, fines, penalties or orders requiring a change in our business practices, which could in the future materially and adversely affect our financial position, results of operations, or cash flows in a particular period.