The use of
monoclonal antibodies (MAbs) directed against
tumor-associated
antigens for targeting of
photosensitizers is an interesting option to improve the selectivity of
photodynamic therapy (
PDT). Hydrophilic
photosensitizers are most suitable for conjugation to MAbs because of their water solubility. The
photosensitizer aluminum (III)
phthalocyanine tetrasulfonate [AlPc(SO3H)4] has many ideal photochemical properties; however, because of its hydrophilicity, the free form of this sensitizer does not readily reach the critical intracellular target and, therefore, is ineffective in
PDT. On the basis of our previous studies, we hypothesized that AlPc(SO3H)4 might be suitable for
PDT when coupled to internalizing
tumor-selective MAbs. In this study, a reproducible procedure is presented for coupling of AlPc(SO3H)4 to MAbs via the tetra-
glycine derivative AlPc(SO2Ngly)4. Conjugation was performed to chimeric MAb (
cMAb) U36 and murine MAbs (mMAb) E48 and 425 using a labile
ester. Conjugates showed preservation of integrity and immunoreactivity and full stability in serum in vitro. At molar ratios >4, the solubility of the conjugates decreased. Data on the in vitro efficacy of
PDT showed that in the chosen experimental setup the internalizing AlPc(SO2Ngly)4-mMAb 425 conjugate was about 7500 times more toxic to A431 cells than the free sensitizer (IC50s, 0.12 nM versus 900 nM). The AlPc(SO2Ngly)4-mMAb 425 conjugate was also more toxic than meta-tetrahydroxyphenylchlorin-mMAb 425 conjugates and free meta-tetrahydroxyphenylchlorin that had been tested previously (M. B. Vrouenraets et al.,
Cancer Res., 59: 1505-1513, 1999) in the same system (IC50s, 7.3 nm and 2.0 nM, respectively). Biodistribution analysis of AlPc(SO2Ngly)4-125I-labeled
cMAb U36 conjugates with different sensitizer:MAb ratios in
squamous cell carcinoma-bearing nude mice revealed selective accumulation in the
tumor, although to a lesser extent than for the unconjugated 125I-labeled
cMAb U36, whereas
tumor:blood ratios were similar. These findings indicate that AlPc(SO3H)4 has high potential for use in
PDT when coupled to internalizing
tumor-selective MAbs.