Development of meta-tetrahydroxyphenylchlorin-monoclonal antibody conjugates for photoimmunotherapy

A limitation of photodynamic therapy is the lack of tumor selectivity of th e photosensitizer, To overcome this problem, a protocol was developed for c oupling of meta-tetrahydroxyphenylchlorin (mTHPC), one of the most promisin g photosensitizers, to tumor-selective monoclonal antibodies (MAbs). mTHPC was radiolabeled with I-131 to facilitate the assessment of the in vitro an d in vivo behavior, After the modification to I-131-mTHPC(CH2COOH)(4), thus increasing the water solubility and creating a functional moiety suitable for coupling, conjugation was performed using a labile ester, Insoluble agg regates were not formed when mTHPC-MAb conjugates with a molar ratio of up to 4 were prepared. These conjugates showed a minimal impairment of the int egrity on SDS-PAGE, full stability in serum in vitro, and an optimal immuno reactivity, To test the in vivo behavior of the mTHPC-MAb conjugates, the head and neck squamous cell carcinoma-selective chimeric MAb U36 was used in head and ne ck squamous cell carcinoma-bearing nude mice. Biodistribution data showed t hat the tumor selectivity of cMAb U36-conjugated mTHPC was increased in com parison with free mTHPC, despite the fact that conjugates with a higher mTH PC:MAb ratio were more rapidly cleared from the blood. Preliminary results on the in vitro efficacy of photodynamic therapy with MAb-conjugated mTHPC showed that mTHPC coupled to the internalizing murine MAb 425 exhibited mor e phototoxicity than when coupled to the noninternalizing chimeric MAb U36.