A novel rapidly internalizing human single chain antibody fragment targeting both epithelioid and sarcomatoid mesothelioma

Objectives The objectives of this study was to evaluate the in vitro cell internalization & in vivo tumor targeting of a novel internalizing human ScFv (M40) on both epithelioid (M-28) & sarcomatoid (VAMT-1) subtypes of mesothelioma.

Methods The M40 was engineered to contain a hexahistidine tag to accommodate ^99mTc-tricarbonyl labeling (^99mTc-M40). For in vitro cell studies, ^99mTc-M40 was incubated at 37^oC for 1&3h with M-28 & VAMT-1 mesothelioma cells & a control cell-line (BPH). For animal studies, ^99mTc-M40 was injected i.v. to athymic mice implanted s.c. with both M-28 and VAMT-1 tumors on the right & left shoulders respectively. A blocking control study with excess unlabeled M40 followed by ^99mTc-M40 was performed. For comparative studies mice bearing both tumors were imaged with μ-SPECT/CT for ^99mTc-M40 or μ-PET/CT for ¹⁸F-FDG & sacrificed at 1, 3 or 6h to assess biodistribution.

Results The ^99mTc-M40 rapidly internalized into both subtypes of tumor cells but not in non-targeted cells & 81-91% of total cell accumulation was accounted for internalization at 37^oC, as early as within 1h. In animal studies, the ^99mTc-M40 was rapidly eliminated from blood & most normal organs (except kidneys), giving tumor uptake >5.84 %ID/g & 4.38 %ID/g for VAMT-1 & M-28 tumors respectively at 3h. In contrast, the control blocking study at 3h exhibited tumor uptake ≤1.6%ID/g & 0.9 %ID/g for VAMT-1 & M-28 tumors respectively. More importantly, in mice bearing both tumors, SPECT/CT using ^99mTc-M40 could image both epitheliod & sarcomatoid mesothelioma whereas the PET/CT using ¹⁸F-FDG could image only sarcomatoid mesothelioma.

Conclusions The ^99mTc-M40 showed rapid internalization & selective tumor targeting in both epithelioid (M-28) & sarcomatoid (VAMT-1) human mesothelioma cancer cells, demonstrating potential use in targeted imaging and therapy.

Research Support NIH/NCI R01CA 135358-01 & IRG-97-150-10/ACS