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Leases
 9 Months Ended
Sep. 30, 2023
Leases [Abstract]  
Leases Leases
The Company’s operating lease activity is comprised of non-cancelable facility leases for office and laboratory space in Boston, Massachusetts.
6 Tide Street Lease
The Company entered into an operating lease for office and laboratory space in Boston, Massachusetts in February 2020 (6 Tide Street Lease), and entered into subsequent amendments through 2023. The amendments run co-terminus with the existing lease. The Company has a total of 23,189 square feet licensed at this facility. The Company has the option to terminate the lease and amendments after November 30, 2023 without penalty. At the adoption of ASC 842, the Company concluded that it is not reasonably certain that it will exercise this option to terminate the lease early.
In January 2023, the Company entered into an amendment to the 6 Tide Street Lease pursuant to which the Company will cease making lease payments for a portion of the leased space no later than November 30, 2023. In September 2023, the Company entered into an additional amendment to the 6 Tide Lease pursuant to which the Company surrendered a portion of the leased space and will cease making lease payments for such portion no later than November 30, 2023. The term for the remainder of the lease will end on November 30, 2025. Following the amendment, the fixed rental payment will be approximately $0.7 million per month through November 30, 2023, and $0.4 million per month after November 30, 2023. Subsequent to the amendment, the Company continues to classify the 6 Tide Street Lease as an operating lease. Upon the lease modification, the Company reassessed its incremental borrowing rate and remeasured the lease liability and right-of-use asset.
In connection with entering into the 6 Tide Street Lease, the Company paid a security deposit of $0.8 million, of which $0.4 million is recorded as a component of other non-current assets and $0.4 million is recorded as a component of other current assets as of September 30, 2023.
IDB Lease
On March 16, 2022, the Company and IDB 17-19 Drydock Limited Partnership, as landlord (Landlord), entered into a lease agreement (IDB Lease) with respect to approximately 81,229 square feet of office and laboratory space (Premises) in Boston, Massachusetts. The initial fixed rental rate is $0.5 million per month, which is for a 12 month period during which the base rent is payable for 65,000 square feet, and will increase 3% per annum thereafter for the entire 81,229 square feet leased.
The accounting commencement date occurred in April 2023 when both the Landlord's build-out and the tenant improvements were substantially completed. On the accounting commencement date, the Company recorded an operating lease right-of-use (ROU) asset of $77.6 million and a total lease liability of $63.6 million.
The IDB Lease has a term of approximately 10 years, unless earlier terminated in accordance with the terms of the IDB Lease. The Company has (i) the option to extend the IDB Lease for an additional period of five (5) years, and (ii) a right of first offer on adjacent space to the Premises, subject to the terms and conditions of the IDB Lease. As these options are not reasonably certain of occurring, they have not be included in the initial calculation of the Company's ROU asset upon lease commencement.

    Under the terms of the IDB Lease, the Landlord provided an allowance of $19.5 million toward the cost of completing tenant improvements for the Premises. In addition, the Landlord provided an additional contribution of $1.6 million toward the cost of tenant improvements to the Premises, which amount shall be repaid by the Company over the term of the IDB Lease. Such repayments are included in the maturity of the lease liability table below.
The Company concluded that the improvements resulting from both the Landlord's build-out and the tenant improvements are the Landlord's assets for accounting purposes. Costs incurred by the Company related to the tenant improvements in excess of the Landlord's allowance were reclassified from other non-current assets to right-of-use assets upon commencement of the IDB lease and will be recognized as rent expense over the remaining lease term. As of September 30, 2023, the Company had received the full tenant improvement allowance from the Landlord.
In connection with the execution of the IDB Lease, the Company executed a cash-collateralized letter of credit, which may be reduced in the future subject to reduction requirements specified in the IDB Lease therein. The cash collateralizing the letter of credit is classified as restricted cash on the Company's condensed consolidated balance sheets.
Summary of all lease costs recognized under ASC 842
The components of operating lease cost were as follows (in thousands):
Three Months Ended September 30, 2023Nine Months Ended September 30, 2023
Operating lease cost$4,064$12,507
Variable lease cost— — 
Total lease cost$4,064$12,507
Supplemental information related to operating leases was as follows:
Other informationNine Months Ended September 30, 2023
Operating cash flows used for operating leases (in thousands)$14,064
Weighted average remaining lease term8.3 years
Weighted average discount rate8.13%
Future payments due under operating leases as of September 30, 2023 were as follows (in thousands):
Maturity of Lease Liability
As of September 30, 2023
2023 (excluding the nine months ended September 30, 2023)
2,206
202413,098
202513,175
20268,826
20279,083
Thereafter50,440
Total lease payments$96,827
Less: imputed interest(27,754)
Present value of operating lease liabilities$69,073
IDB Sublease
In December 2022, the Company entered into a sublease agreement to sublease a portion of the office and laboratory space leased under the IDB Lease to a third-party (subtenant). The sublease term is 3 years and the subtenant has an option to extend the lease term for 6 months. The initial fixed rental rate is approximately $0.2 million per month, and will increase 3% per annum thereafter. The sublessee is obligated to pay its ratable portion of operating expenses during the sublease term. The Company received a letter of credit of $0.5 million in place of a security deposit. As of September 30, 2023, no amounts have been drawn on the letter of credit.
The sublease accounting commencement date occurred in April 2023. During the three and nine months ended September 30, 2023, the Company recognized $0.5 million and $1.0 million, respectively, of sublease income. Such amount is recorded as a reduction to rent expense
Leases Leases
The Company’s operating lease activity is comprised of non-cancelable facility leases for office and laboratory space in Boston, Massachusetts.
6 Tide Street Lease
The Company entered into an operating lease for office and laboratory space in Boston, Massachusetts in February 2020 (6 Tide Street Lease), and entered into subsequent amendments through 2023. The amendments run co-terminus with the existing lease. The Company has a total of 23,189 square feet licensed at this facility. The Company has the option to terminate the lease and amendments after November 30, 2023 without penalty. At the adoption of ASC 842, the Company concluded that it is not reasonably certain that it will exercise this option to terminate the lease early.
In January 2023, the Company entered into an amendment to the 6 Tide Street Lease pursuant to which the Company will cease making lease payments for a portion of the leased space no later than November 30, 2023. In September 2023, the Company entered into an additional amendment to the 6 Tide Lease pursuant to which the Company surrendered a portion of the leased space and will cease making lease payments for such portion no later than November 30, 2023. The term for the remainder of the lease will end on November 30, 2025. Following the amendment, the fixed rental payment will be approximately $0.7 million per month through November 30, 2023, and $0.4 million per month after November 30, 2023. Subsequent to the amendment, the Company continues to classify the 6 Tide Street Lease as an operating lease. Upon the lease modification, the Company reassessed its incremental borrowing rate and remeasured the lease liability and right-of-use asset.
In connection with entering into the 6 Tide Street Lease, the Company paid a security deposit of $0.8 million, of which $0.4 million is recorded as a component of other non-current assets and $0.4 million is recorded as a component of other current assets as of September 30, 2023.
IDB Lease
On March 16, 2022, the Company and IDB 17-19 Drydock Limited Partnership, as landlord (Landlord), entered into a lease agreement (IDB Lease) with respect to approximately 81,229 square feet of office and laboratory space (Premises) in Boston, Massachusetts. The initial fixed rental rate is $0.5 million per month, which is for a 12 month period during which the base rent is payable for 65,000 square feet, and will increase 3% per annum thereafter for the entire 81,229 square feet leased.
The accounting commencement date occurred in April 2023 when both the Landlord's build-out and the tenant improvements were substantially completed. On the accounting commencement date, the Company recorded an operating lease right-of-use (ROU) asset of $77.6 million and a total lease liability of $63.6 million.
The IDB Lease has a term of approximately 10 years, unless earlier terminated in accordance with the terms of the IDB Lease. The Company has (i) the option to extend the IDB Lease for an additional period of five (5) years, and (ii) a right of first offer on adjacent space to the Premises, subject to the terms and conditions of the IDB Lease. As these options are not reasonably certain of occurring, they have not be included in the initial calculation of the Company's ROU asset upon lease commencement.

    Under the terms of the IDB Lease, the Landlord provided an allowance of $19.5 million toward the cost of completing tenant improvements for the Premises. In addition, the Landlord provided an additional contribution of $1.6 million toward the cost of tenant improvements to the Premises, which amount shall be repaid by the Company over the term of the IDB Lease. Such repayments are included in the maturity of the lease liability table below.
The Company concluded that the improvements resulting from both the Landlord's build-out and the tenant improvements are the Landlord's assets for accounting purposes. Costs incurred by the Company related to the tenant improvements in excess of the Landlord's allowance were reclassified from other non-current assets to right-of-use assets upon commencement of the IDB lease and will be recognized as rent expense over the remaining lease term. As of September 30, 2023, the Company had received the full tenant improvement allowance from the Landlord.
In connection with the execution of the IDB Lease, the Company executed a cash-collateralized letter of credit, which may be reduced in the future subject to reduction requirements specified in the IDB Lease therein. The cash collateralizing the letter of credit is classified as restricted cash on the Company's condensed consolidated balance sheets.
Summary of all lease costs recognized under ASC 842
The components of operating lease cost were as follows (in thousands):
Three Months Ended September 30, 2023Nine Months Ended September 30, 2023
Operating lease cost$4,064$12,507
Variable lease cost— — 
Total lease cost$4,064$12,507
Supplemental information related to operating leases was as follows:
Other informationNine Months Ended September 30, 2023
Operating cash flows used for operating leases (in thousands)$14,064
Weighted average remaining lease term8.3 years
Weighted average discount rate8.13%
Future payments due under operating leases as of September 30, 2023 were as follows (in thousands):
Maturity of Lease Liability
As of September 30, 2023
2023 (excluding the nine months ended September 30, 2023)
2,206
202413,098
202513,175
20268,826
20279,083
Thereafter50,440
Total lease payments$96,827
Less: imputed interest(27,754)
Present value of operating lease liabilities$69,073
IDB Sublease
In December 2022, the Company entered into a sublease agreement to sublease a portion of the office and laboratory space leased under the IDB Lease to a third-party (subtenant). The sublease term is 3 years and the subtenant has an option to extend the lease term for 6 months. The initial fixed rental rate is approximately $0.2 million per month, and will increase 3% per annum thereafter. The sublessee is obligated to pay its ratable portion of operating expenses during the sublease term. The Company received a letter of credit of $0.5 million in place of a security deposit. As of September 30, 2023, no amounts have been drawn on the letter of credit.
The sublease accounting commencement date occurred in April 2023. During the three and nine months ended September 30, 2023, the Company recognized $0.5 million and $1.0 million, respectively, of sublease income. Such amount is recorded as a reduction to rent expense