Note 6. Commitments and Contingencies Operating Leases The Company leases office space under various operating leases. Total rental expense for all operating leases in the accompanying condensed consolidated statements of operations and comprehensive loss was $42,000 and $141,000 for the three months ended June 30, 2018 and 2017, respectively, and $84,000 and $182,000 for the six months ended June 30, 2018 and 2017, respectively. In February 2015, Tokai entered into a sublease for 15,981 square feet of office space in Boston, Massachusetts. The term of the sublease commenced on April 1, 2015 and expired on December 31, 2016 and was subsequently extended through July 31, 2018. In November 2017, the Company terminated the lease early and paid an additional $455,000 in advance rent in conjunction with the lease termination. In September 2015, Otic entered into a three-year operating lease for 5,197 square feet of office space in Irvine, California. The lease had an expiration date of August 31, 2018; however, the Company extended the term of the lease through September 30, 2021 by amending the office lease in April 2018. Future payments under noncancelable extended operating leases having initial or remaining terms of one year or more are as follows for the remaining fiscal year and thereafter (in thousands): 2018 (remainder of) $ 88 2019 187 2020 195 2021 152 Total minimum lease payments $ 622 Restricted Cash and Letter of Credit The Company was required to maintain a letter of credit totaling $70,000 for the benefit of the landlord of Tokais Boston office. The landlord could draw against the letter of credit in the event of default by the Company. The Company held $70,000 in restricted cash as part of current assets on the condensed consolidated balance sheet as of December 31, 2017. Although the Boston office lease was terminated in November 2017, the process to release the restricted cash was not completed as of December 31, 2017. On March 12, 2018, the restricted cash was released and transferred into general