Commitments and contingencies Commitments. We are obligated to make future minimum payments under operating leases for building space used for general office and research and development purposes. The leases expire between 2019 through 2029 and include options to renew at the then fair market rental for the facilities. The lease agreements typically contain scheduled rent increases and provide for tenant improvement allowances. Assuming we do not exercise any extensions, future minimum lease payments under all non-cancelable operating leases are set forth below. In addition, we have certain non-cancelable obligations under other agreements, including supply agreements relating to the manufacture of ADCETRIS and our product candidates which contain annual minimum purchase commitments and other firm commitments when a binding forecast is provided. As of December 31, 2018 , our future obligations related to building leases and supply and other agreements are as follows (in thousands): Building Leases Supply and Other Agreements Years ending December 31, 2019 $10,332 $92,105 2020 11,863 30,689 2021 12,770 31,091 2022 12,288 24,033 2023 12,142 21,720 Thereafter 30,517 43,440 Total $89,912 $243,078 Rent expense attributable to non-cancelable operating leases totaled approximately $8.7 million , $6.6 million , and $5.6 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. We entered into various operating leases for building space in 2018, with lease commencement dates in 2019. Future obligations related to those leases are included in the table above but no rent expense has been recognized in 2018. Non-cancelable obligations under other agreements do not include payments that are contingent upon achievement of certain progress-dependent milestones, as well as the payment of royalties based on net sales of commercial products. These amounts have been excluded from the table because the events triggering the obligations have not yet occurred. Contingencies. On March 29, 2017, a stockholder derivative lawsuit, or the Stockholder Derivative Action, was filed in Washington Superior Court for the County of Snohomish, or the Snohomish County Superior Court. The complaint named as defendants certain of our current and former executives and members of our board of directors. We were named as a nominal defendant. The Stockholder Derivative Action was largely based on a January 2017 securities class action suit, or the CD33A Class Action, that was filed in the United States District Court for the Western District of Washington and which alleged material misrepresentations and omissions in public statements regarding our business, operational and compliance policies, as well as violations of Sections 10(b) and 20(b) of the Securities Exchange Act. Following the dismissal with prejudice of the CD33A Class Action, on August 30, 2018, the plaintiffs in the Stockholder Derivative Action filed an amended complaint. This complaint alleged that the defendants breached their fiduciary duties by making or failing to correct certain allegedly improper public statements regarding our former SGN-CD33A program and by wasting our assets by allowing certain SGN-CD33A trials to continue. On October 4, 2018, we filed a motion to dismiss the amended complaint for failure to plead demand futility. On November 15, 2018, the plaintiffs voluntarily dismissed their complaint with prejudice as to plaintiffs. On March 8, 2018, three purported stockholders of Cascadian filed a Verified Complaint to Compel Inspection of Books and Records under 8 Del. C. 220 in the Delaware Court of Chancery against Cascadian, seeking to inspect books and records in order to determine whether wrongdoing or mismanagement has taken place such that it would be appropriate to file claims for breach of fiduciary duty, and to investigate the independence and disinterestedness of the former Cascadian directors with respect to the Cascadian Acquisition. We filed our answer to this complaint on March 28, 2018. As a result of this lawsuit, we may incur litigation and indemnification expenses. In addition, from time to time in the ordinary course of business we become involved in various lawsuits, claims and proceedings relating to the conduct of our business, including those pertaining to the defense and enforcement of our patent or other intellectual property rights. These proceedings are costly and time consuming. Additionally, successful challenges to our patent or other intellectual property rights through these proceedings could result in a loss of rights in the relevant jurisdiction and may allow third parties to use our proprietary technologies without a license from us or our collaborators.