Achilles Therapeutics plc presented a patient case study from the Company’s ongoing Phase I/IIa THETIS trial in metastatic or recurrent melanoma at the American Association for Cancer Research (AACR) Annual Meeting 2021. Through a comprehensive translational research program, data from Patient T-05 offer insight into the in vivo dynamics of clonal neoantigen T cells (cNeT) post-dosing and the potential to develop a potency-based release assay. In a case study from the ongoing THETIS trial, a patient diagnosed with cutaneous melanoma received a three-cycle combination of ipilimumab and nivolumab which was stopped due to toxicity. In 2020, the patient had a recurrent lesion excised and cNeT manufactured through the VELOSTM process. The specificity and fitness of the cNeT were measured by flow cytometric analysis of IFN-? and TNF-a cytokines, markers of T cell activity. Up to 53% of the T cells in the manufactured product were reactive to the patient’s own clonal neoantigens following stimulation. cNeT can be tracked after dosing by using peptides that incorporate patient-specific clonal mutations from the tumor to stimulate cells from the blood of the patient. By using ELISpot technology to detect cNeT that produce cytokine in response to this stimulus, the expansion and persistence of cNeT in circulation can be calculated.