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 was $7.6 million, $7.7 million and $7.7 million for the years ended December 31, 2023, 2022 and 2021, respectively. Sublease income is included in other income (expense) in our 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):


	Year Ended December 31,
	2023		2022		2021
Operating lease expense	$	5,563	$	6,222	$	6,265

Cash paid for amounts included in the measurement of lease liabilities for the years ended December 31, 2023, 2022, and 2021 was $7.2 million, $6.8 million, and $7.0 million, respectively and were included in change in lease liabilities in our consolidated statement of cash flows.

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


	December 31,
	2023		2022
Operating lease liabilities:
Current portion of lease liabilities (included in other current liabilities)	$	4,496	$	3,631
Long-term portion of lease liabilities		29,720		32,801
Total operating lease liabilities	$	34,216	$	36,432

As of December 31, 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
2024	$	5,684	$	7,605
2025		5,854		6,980
2026		6,030		6,122
2027		6,211		6,052
2028		6,397		6,215
Thereafter		15,103		15,062
Total	$	45,279		48,036
Less:
Present value adjustment				(13,820	)
Total			$	34,216

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


	December 31,
	2023			2022
Weighted average remaining lease term	6.7 years			7.6 years
Weighted average discount rate		10.1	%		10.1	%

As of December 31, 2023, our material non-cancelable purchase and other commitments for the supply of HEPLISAV-B totaled $43.4 million. The following summarizes our material purchase commitments at December 31, 2023 and the effect those obligations are expected to have on our liquidity and cash flows in future periods (in thousands):


Years ending December 31,	(in thousands)
2024	$	18,866
2025		11,983
2026		12,516
2027		-
2028		-
Thereafter		-
Total	$	43,365

On September 7, 2023 (the “Effective Date”), we entered into an agreement (the “Avecia Supply Agreement”) with Nitto Denko Avecia Inc. (“Avecia”) for the manufacture and supply of our CpG 1018 adjuvant using a specific production process. Under the Avecia Supply Agreement, Avecia has agreed to produce and supply to us quantities of CpG 1018 adjuvant ordered by us after the Effective Date. Subject to certain conditions in the Avecia Supply Agreement, we are obligated to purchase all of our annual volume requirements of CpG 1018 adjuvant from Avecia up to a specified production capacity. We may alternatively order CpG 1018 adjuvant produced using a different production process pursuant to the existing supply agreement between us and Avecia dated October 1, 2012 (the “2012 Agreement”). As of December 31, 2023, our aggregate minimum commitment for the supply of CpG 1018 adjuvant under the Avecia Supply Agreement was $7.4 million within the next 12 months. As of December 31, 2022, we had no non-cancelable purchase and other commitments for the supply of CpG 1018 adjuvant.

In addition to the non-cancelable commitments included above, we have entered into contractual arrangements that obligate us to make payments to the contractual counterparties upon the occurrence of future events. In addition, in the normal course of operations, we have entered into license and other agreements and intend to continue to seek additional rights relating to compounds or technologies in connection with our discovery, manufacturing and development programs. Under the terms of the agreements, we may be required to pay future up-front fees, milestones and royalties on net sales of products originating from the licensed technologies, if any, or other payments contingent upon the occurrence of future events that cannot reasonably be estimated.

We also rely on and have entered into agreements with research institutions, contract research organizations and clinical investigators as well as clinical material manufacturers. These agreements are terminable by us upon written notice. Generally, we are liable only for actual effort expended by the organizations at any point in time during the contract through the notice period.

As of December 31, 2023, the aggregate principal amount of our convertible senior notes ("Convertible Notes") was $225.5 million, excluding debt discount of $2.8 million (See Note 10).

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 December 30, 2023 and was collateralized by a certificate of deposit for €0.2 million, which has been included in restricted cash in the consolidated balance sheets as of December 31, 2023 and 2022.

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, May 2022 and September 2023, we received $1.0 million from TriSalus because it met pre-commercialization milestones. We recorded the proceeds as gain on sale of assets in our consolidated statements of operations. We paid Holdings $0.5 million in each of September 2021, May 2022 and October 2023. We included the payments in selling, general and administrative expenses in our consolidated statements of operations.

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.