1 1 . Contractual Obligations and Commitments Operating Lease Corporate Headquarters Lease On April 5, 2018, we entered into an operating lease agreement for 30,175 square feet of office space located in Boston, Massachusetts for our corporate headquarters. The lease commencement date was August 15, 2018 and we took occupancy in September 2018. We are leasing this space under a non-cancelable operating lease with an initial term ending after 123 months and an option to extend the lease for an additional five-year term. We did not include the extension option in our right-of-use asset and lease liability calculation as we did not consider it reasonably certain that we would exercise the option. Under the lease agreement, we received a three-month free rent period, which commenced on August 15, 2018, and a tenant improvement allowance up to $3.8 million. We provided the lessor with a letter of credit to secure its obligations under the lease in the initial amount of $2.4 million, to be reduced to $1.8 million on the third anniversary of the rent commencement date and to $1.2 million on the fifth anniversary of the rent commencement date if we meet certain conditions set forth in the lease at each such time. The letter of credit amount is included in deposits and other assets on the accompanying condensed consolidated balance sheets. Ionis Sublease On November 12, 2018 we entered into an operating lease agreement with Ionis Pharmaceuticals to sublease 4,723 square feet of office space located in Carlsbad, California. The commencement date was March 2018 and the term of the lease is 64 months with a four-month free rent period. Cambridge Lease We leased office space in a building in Cambridge, Massachusetts under an operating lease that commenced in April 2015 and was subsequently amended and expanded in February 2016 and March 2017. The lease was scheduled to expire in April 2020. We have subsequently terminated the lease effective April 2019. When we determined our lease term for our operating lease right-of-use assets and lease liabilities for these leases we did not include extension option as we are not reasonably certain we will exercise the option. Other Operating Lease Information Other information related to our operating leases were as follows (dollar amounts in thousands): At March 31, 2019 Right-of-use operating lease assets $ 11,602 Operating lease liabilities (1) $ 16,324 Weighted average remaining lease term 9.5 years Weighted average discount rate 8 % (1) Current portion of $1.3 million included in other current liabilities and the difference of $15 million included in long-term portion of lease liabilities on our condensed consolidated balance sheet. Annual maturities of our operating lease liabilities as of March 31, 2019 are as follows (in thousands): Operating Leases Remainder of 2019 Years ended December 31, $ 1,921 2020 2,422 2021 2,370 2022 2,403 2023 2,400 Thereafter 11,961 Total minimum lease payments $ 23,477 Less: Imputed interest (7,153 ) Total operating lease liability $ 16,324 Operating lease expense was $0.5 million and $0.2 million for the three months ended March 31, 2019 and 2018, respectively. Cash paid for amounts included in the measurement of lease liabilities for the three months ended March 31, 2019 was $0.6 million and was included in net cash used in operating activities in our condensed consolidated statements of cash flows. Purchase Commitments Purchase commitments include agreements to purchase goods or services that are enforceable and legally binding on us and that specify all significant terms, including, fixed or minimum quantities to be purchased; fixed, minimum or variable price provisions; and the approximate timing of the transaction. Such obligations are related principally to inventory purchase orders based on our current manufacturing needs and require significant lead times to be fulfilled by our vendors. Purchase commitments exclude agreements that are cancelable without penalty. The following table summarizes the purchase commitments as of March 31, 2019 (in thousands): Payments due by period Year Ending December 31, Total Less than 1 year 1 to 2 years Purchase commitments 7,173 5,811 1,362 Total $ 7,173 $ 5,811 $ 1,362