Total Tumor Uptake and Absorbed Dose of 177Lu-Lilotomab Satetraxetan in a First in Human Trial for Relapsed Non-Hodgkin Lymphoma - Are We Hitting the Target?
Authors: Løndalen Ayca , Blakkisrud Johan , Revheim Mona-Elisabeth , Dahle Jostein , Kolstad Arne , Stokke Caroline 
Published: April 2021

Abstract
Purpose: 177Lu-lilotomab satetraxetan, a novel CD37 directed radioimmunotherapy (RIT), has been investigated in a first-in-human phase 1/2a study for relapsed non-Hodgkin lymphoma (NHL). Absorbed dose for all tumor tissue in the body is a crucial parameter in RIT which has traditionally been challenging to calculate. The aim of this study was to investigate the correlations between baseline FDG PET/CT and posttreatment SPECT/CT parameters, absorbed dose-response relationships and clinical responses.Materials and methods: A total of 15 patients with different pre-treatment and pre-dosing regimens were included. 177Lu-lilotomab satetraxetan was administered at dosage levels of 10, 15 or 20 MBq/kg. Total radioimmunoconjugate tumor volume (tRTV), total radioimmunoconjugate lesion uptake (tRLU) and total tumor absorbed dose (tTAD) were calculated from posttreatment SPECT/CT. The measured uptake values and absorbed doses were normalized for dosage when appropriate. For some of the analyses, the cohort was divided into low (arm 1) and high (arm 4+5) non-radioactive lilotomab pre-dosing groups. tMTV and tTLG were calculated from FDG PET/CT performed at baseline, 3 and 6 months after RIT, and the percent change for these parameters calculated (∆tMTV3months, ∆tTLG3months and ∆tMTV6months, ∆tTLG6months). Clinical responses were evaluated at 6 months.Results: tTMV and tRTV were significantly correlated (p<0.01). A correlation was also found between tTLG and tRLU (p<0.01). Correlations were not observed between baseline tTMV and tTAD. Decreases in ∆tMTV and ∆tTLG were significantly higher at PET3months for patients receiving tTAD≥200cGy compared to patients receiving lower tumor absorbed doses (p=.03 for both). Also, significant decreases in ∆tMTV3months, ∆tTLG3months and ∆tMTV6months, ∆tTLG6months were observed with increasing tTAD in the high lilotomab patient group. Similarly, responders (patients with complete remission and partial remission) had higher mean tTAD compared to non-responders (stable disease and progressive disease). This was statistically significant in the high lilotomab group. Across the entire population, all non-responders had tTAD < 200cGy, and all patients with tTAD ≥ 200cGy were responders.Conclusion: This work indicates that 177Lu-lilotomab satetraxetan targets FDG avid lesions, and that increasing baseline (tMTV) does not have a decreasing effect on the total tumor absorbed dose (tTAD). The patient group receiving a higher amount of lilotomab pre-dosing demonstrated an absorbed dose–response relationship. Similar results were not observed in the low lilotomab group, which were expected since an overall very good response rate could mask such a relationship for this group. Regardless of pre-dosing, a mean absorbed dose to the total tumor tissue (tTAD) limit of 200cGy may prove valuable to separate clinical non-responders from responders.