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 renewal options at our election to renew or extend the lease for two successive five-year terms. These optional periods have not been considered in the determination of the ROU 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 $7.7 million for each of the years ended

December 31, 2022, 2021 and 2020. 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,
	2022		2021		2020
Operating lease expense	$	6,222	$	6,265	$	6,267

Cash paid for amounts included in the measurement of lease liabilities for the years ended December 31, 2022, 2021, and 2020 was $6.8 million, $7.0 million, and $6.9 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, 2022		December 31, 2021
Operating lease liabilities:
Current portion of lease liabilities (included in other current liabilities)	$	3,631	$	2,577
Long-term portion of lease liabilities		32,801		34,316
Total operating lease liabilities	$	36,432	$	36,893

At December 31, 2022, 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	$	5,518	$	6,943
2024		5,684		7,073
2025		5,854		6,448
2026		6,030		5,875
2027		6,211		6,033
Thereafter		21,500		21,203
Total	$	50,797		53,575
Less:
Present value adjustment				(17,143	)
Total			$	36,432

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


	December 31, 2022			December 31, 2021
Weighted average remaining lease term	7.6 years			9.1 years
Weighted average discount rate		10.1	%		10.1	%

As of December 31, 2022, our purchase commitments include non-cancelable purchase commitments for the supply of HEPLISAV-B. As of December 31, 2022, we have no material non-cancelable purchase and other commitments, for the

supply of CpG 1018 adjuvant. The following summarizes our material purchase commitments at December 31, 2022 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)
2023	$	9,312
2024		10,857
2025		11,367
2026		11,872
2027		—
Thereafter		—
Total	$	43,408

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, 2022, the aggregate principal amount of our convertible senior notes due 2026 ("Convertible Notes") was $225.5 million, excluding debt discount of $3.9 million (See Note 10). The Convertible Notes mature on May 15, 2026, unless converted, redeemed or repurchased in accordance with their terms prior to such date.

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 31, 2022 and is 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, 2022 and 2021.

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 the years ended December 31, 2022 and 2021, 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 consolidated statements of operations in the years ended December 31, 2022 and 2021. We also paid Holdings $0.5 million in each of years ended December 31, 2022 and 2021. We included such payments in selling, general and administrative expenses in our consolidated statements of operations. No liability has been recorded under this agreement as of December 31, 2022.

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, financial statements, results of operations, or cash flows in a particular period.