Dosimetry of [Pb]VMT01, a MC1R-Targeted Alpha Therapeutic Compound, and Effect of Free Tl on Tissue Absorbed Doses.

[Pb]VMT01 is a melanocortin 1 receptor (MC1R) targeted theranostic ligand in clinical development for alpha particle therapy for melanoma. Pb has an elementally matched gamma-emitting isotope Pb; thus, [Pb]VMT01 can be used as an imaging surrogate for [Pb]VMT01. [Pb]VMT01 human serum stability studies have demonstrated retention of the Bi daughter within the chelator following beta emission of parent Pb. However, the subsequent alpha emission from the decay of Bi into Tl results in the generation of free Tl. Due to the 10.64-hour half-life of Pb, accumulation of free Tl in the injectate will occur. The goal of this work is to estimate the human dosimetry for [Pb]VMT01 and the impact of free Tl in the injectate on human tissue absorbed doses. Human [Pb]VMT01 tissue absorbed doses were estimated from murine [Pb]VMT01 biodistribution data, and human biodistribution values for Tl chloride (a cardiac imaging agent) from published data were used to estimate the dosimetry of free Tl. Results indicate that the dose-limiting tissues for [Pb]VMT01 are the red marrow and the kidneys, with estimated absorbed doses of 1.06 and 8.27 mGy/MBq. The estimated percent increase in absorbed doses from free Tl in the injectate is 0.03% and 0.09% to the red marrow and the kidneys, respectively. Absorbed doses from free Tl result in a percent increase of no more than 1.2% over [Pb]VMT01 in any organ or tissue. This latter finding indicates that free Tl in the [Pb]VMT01 injectate will not substantially impact estimated tissue absorbed doses in humans.