Commitments - Additional Information (Detail)												1 Months Ended			12 Months Ended
	Mar. 25, 2021 USD ($)	Jul. 27, 2018 USD ($) Lease	Jun. 07, 2018 $ / shares shares	Mar. 01, 2018	Aug. 24, 2017 USD ($)	Dec. 05, 2016 USD ($) shares	Mar. 22, 2016 USD ($)	Sep. 28, 2015 $ / shares shares	Apr. 24, 2015 $ / shares shares	Feb. 04, 2015 EUR (€) Program	Feb. 04, 2015 USD ($) Program	Dec. 31, 2018 EUR (€)	Apr. 30, 2017 USD ($)	Feb. 14, 2017 USD ($)	Dec. 31, 2018 EUR (€) Lease shares	Dec. 31, 2018 USD ($) Lease shares	Jan. 31, 2018 USD ($)	Dec. 31, 2017 EUR (€) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2015	Dec. 05, 2019 USD ($)	Dec. 31, 2018 USD ($)	Oct. 01, 2017	Jan. 01, 2016 USD ($)
Other Commitments [Line Items]
Research and development expenses																$ 33,620,223			$ 22,359,712
General and administrative expense																$ 44,483,938			$ 9,325,017
Ordinary share | shares															2,334,285	2,334,285		611,933	611,933
Service Agreements [Member]
Other Commitments [Line Items]
Employees aggregate compensation | €															€ 300,000
Research and development expenses															1,001,000	$ 1,334,000		€ 724,000	$ 933,000
Future obligations under agreement												€ 150,333			€ 150,333							$ 192,000
Employment Agreements [Member]
Other Commitments [Line Items]
Employees aggregate compensation																	$ 710,000
Future obligations under agreement																						1,258,750
Agreegate discretionary bonus																3,415,000	$ 1,196,000		850,000
Employee agreement description																	The employment agreements also provide for an annual guaranteed cash bonus targeted at 100% of annual compensation. The agreements also provide for discretionary annual performance bonuses targeted to be not less than 50-60% of the employee's base salary and grants of restricted stock.
Employment agreement term																	3 years
Percentage of fully-diluted outstanding shares			5.00%	3.00%
Consulting And Other Agreements [Member]
Other Commitments [Line Items]
General and administrative expense																263,970			351,960
Research Agreement One [Member]
Other Commitments [Line Items]
Research and development expenses																$ 418,000			418,000
Consulting agreement term							5 years
Quarterly payment received for Research and development agreement							$ 104,500
Restricted Shares [Member]
Other Commitments [Line Items]
Employees share award | shares			1,306,348												1,306,348	1,306,348
Employees share award, per share | $ / shares			$ 15.00
Ordinary shares forfeiture period																				3 years
Restricted Shares [Member] | Service Agreements [Member]
Other Commitments [Line Items]
Employees share award | shares									2,704,800
Employees share award, per share | $ / shares									$ 7.76
Ordinary shares forfeiture period									3 years
Restricted Shares [Member] | Consulting And Other Agreements [Member]
Other Commitments [Line Items]
Employees share award | shares								662,910
Ordinary shares forfeiture period								3 years
Restricted Shares [Member] | Director [Member] | Consulting And Other Agreements [Member]
Other Commitments [Line Items]
Consulting agreement term								3 years
Maximum [Member] | Service Agreements [Member]
Other Commitments [Line Items]
Employees aggregate compensation															€ 410,000	$ 522,000
Maximum [Member] | Employment Agreements [Member]
Other Commitments [Line Items]
Employees aggregate compensation																	$ 1,075,000
Percentage of fully-diluted outstanding shares															8.00%	8.00%
General and Administrative Expense [Member] | Restricted Shares [Member] | Consulting And Other Agreements [Member]
Other Commitments [Line Items]
Employees share award, per share | $ / shares								$ 7.72
U.S. Department Of Health And Human Services [Member] | Research Agreement [Member]
Other Commitments [Line Items]
Research and development expenses																$ 111,938			115,374
Quarterly payment received for Research and development agreement													$ 21,250
Agreement amended date									Mar. 25, 2016
Agreement expiration date									Apr. 30, 2017
Research agreement payment received period									3 years
Cornell University [Member] | Research Agreement [Member]
Other Commitments [Line Items]
Research and development expenses																143,073			698,307
Agreement amended date						Jun. 12, 2017
Research and collaboration agreement period						3 years
Ordinary share | shares						25,000
Research and collaboration agreement period														1 year
Research and collaboration agreement amount														$ 679,473
Cornell University [Member] | Issued at Closing of Merger [Member] | Research Agreement [Member]
Other Commitments [Line Items]
Research and development expenses						$ 17,000										1,625,152			1,029,904
Cornell University [Member] | Issued Following Closing of Merger [Member] | Research Agreement [Member]
Other Commitments [Line Items]
Research and development amended agreement amount					$ 182,520
National Institutes of Health [Member] | License Agreement [Member]
Other Commitments [Line Items]
Research and development expenses																50,000			0
BRI-Alzan [Member] | License Agreement [Member]
Other Commitments [Line Items]
Research and development expenses																15,000			30,000
Sales milestone payments																								$ 4,500,000
Scenario, Forecast [Member] | U.S. Department Of Health And Human Services [Member] | Research Agreement [Member]
Other Commitments [Line Items]
Future obligations under agreement	$ 191,250
Agreement expiration date	Mar. 25, 2021
Scenario, Forecast [Member] | Cornell University [Member] | Research Agreement [Member]
Other Commitments [Line Items]
Future obligations under agreement																					$ 2,002,228
License Agreements Effective February 4, 2015 [Member] | UCL Business, PLC [Member] | License Agreement [Member]
Other Commitments [Line Items]
Research and development expenses																75,073			73,250
Number of collaborations programs | Program										8	8
Sales milestone payments										€ 39,800,000												50,700,000
Maintenance fee										€ 50,000	$ 64,000
License Agreement Effective July 28, 2017 [Member] | UCL Business, PLC [Member] | License Agreement [Member]
Other Commitments [Line Items]
Research and development expenses																66,630			82,260
License Agreements Effective March 15, 2018 [Member] | UCL Business, PLC [Member] | License Agreement [Member]
Other Commitments [Line Items]
Research and development expenses																133,728			0
July 27, 2018 Lease [Member] | London UK [Member]
Other Commitments [Line Items]
Operating lease rent expense																148,561			0
Annual base rent		$ 363,000
Operating lease termination date		May 24, 2027
Number of leases expired | Lease		2
Number of new leases | Lease		2
February,2016 Lease [Member] | London UK [Member]
Other Commitments [Line Items]
Operating lease expiration												2021-02
Operating lease rent expense																$ 273,430			279,303
October 1,2017 Lease [Member] | Leiden Netherland [Member]
Other Commitments [Line Items]
Operating lease expiration															2018-09	2018-09
Operating lease rent expense																$ 9,313			5,273
Term of non-cancellable operating lease															1 year	1 year
October 1,2017 Lease [Member] | Leiden Netherland [Member] | Minimum [Member]
Other Commitments [Line Items]
Operating lease, renewal Term																							1 year
October 1,2017 Lease [Member] | Leiden Netherland [Member] | Maximum [Member]
Other Commitments [Line Items]
Operating lease, renewal Term																							4 years
June 2017 Lease [Member] | London UK [Member]
Other Commitments [Line Items]
Operating lease expiration															2018-07	2018-07
Operating lease rent expense																$ 73,846			$ 85,222
Deferred rent																						$ 0
Operating leases rent expense payment description															The Company records monthly rent expense on a straight-line basis from June 1, 2017 through July 23, 2018.	The Company records monthly rent expense on a straight-line basis from June 1, 2017 through July 23, 2018.
Number of non cancellable operating leases | Lease															2	2