FDG PET/CT and Dosimetric Studies of 177 Lu-Lilotomab Satetraxetan in a First-in-Human Trial for Relapsed Indolent non-Hodgkin Lymphoma—Are We Hitting the Target?

Purpose [ 177 Lu]Lu-lilotomab satetraxetan, a novel CD37 directed radioimmunotherapy (RIT), has been investigated in a first-in-human phase 1/2a study for relapsed indolent non-Hodgkin lymphoma. In this study, new methods were assessed to calculate the mean absorbed dose to the total tumor volume, with the aim of establishing potential dose–response relationships based on 2-deoxy-2-[18F]fluoro- d -glucose (FDG) positron emission tomography (PET) parameters and clinical response. Our second aim was to study if higher total tumor burden induces reduction in the 177 Lu-lilotomab satetraxetan accumulation in tumor. Procedures Fifteen patients with different pre-dosing (non-radioactive lilotomab) regimens were included and the cohort was divided into low and high non-radioactive lilotomab pre-dosing groups for some of the analyses. 177 Lu-lilotomab satetraxetan was administered at dosage levels of 10, 15, or 20 MBq/kg. Mean absorbed doses to the total tumor volume ( tTAD ) were calculated from posttreatment single-photon emission tomography (SPECT)/computed tomography (CT) acquisitions. Total values of metabolic tumor volume ( tMTV ), total lesion glycolysis ( tTLG ) and the percent change in these parameters were calculated from FDG PET/CT performed at baseline, and at 3 and 6 months after RIT. Clinical responses were evaluated at 6 months as complete remission (CR), partial remission (PR), stable disease (SD), or progressive disease (PD). Results Significant decreases in tMTV and tTLG were observed at 3 months for patients receiving tTAD ≥ 200 cGy compared to patients receiving tTAD < 200 cGy ( p = .03 for both). All non-responders had tTAD < 200 cGy. Large variations in tTAD were observed in responders. Reduction in 177 Lu-lilotomab satetraxetan uptake in tumor volume was not observed in patients with higher baseline tumor burden (tTMV). Conclusion tTAD of ≥ 200 cGy may prove valuable to ensure clinical response, but further studies are needed to confirm this in a larger patient population. Furthermore, this work indicates that higher baseline tumor burden (up to 585 cm 3 ) did not induce reduction in radioimmunoconjugate accumulation in tumor.