CARBON‐14 LABELING AND BIOLOGICAL ACTIVITY OF THE TUMOR‐LOCALIZING DERIVATIVE OF HEMATOPORPHYRIN

Abstract— Carbon‐14‐labeled hematoporphyrin ([¹⁴C]HP) was synthesized by two methods, (i) Using an in vitro avian whole‐blood system, [¹⁴C]protoheme was obtained biosynthetically by incorporating [⁴C]aminolevulinic acid into the porphyrin ring structure. Subsequently, the [¹⁴C]protoheme was converted to [⁴C]HP by standard procedures, (ii) By adopting several well‐characterized chemical reactions, deuteroporphyrin was treated with [¹⁴C]acetyl chloride, giving [¹⁴C]diacetyl deuteroporphy‐rin which was readily reduced and hydrolyzed to [¹⁴C]HP (with thecarbon–14 label on the hydroxyethyl side‐chains). These two methods are simple and afford good yields of [¹⁴C]HP with moderate to high specific activities. The [¹⁴C]HP was then treated with acetic acid/sulfuric acid followed by sodium hydroxide to give [¹⁴C]HPD. Upon gel‐ and ultra‐filtration, the [¹⁴C]HPD was enriched in the so‐called tumor‐localizing fraction of HPD, giving [¹⁴C]PII with specific activities of 0.4 Ci/mol (biosynthesis) and 10 Ci/mol (chemical synthesis). These [¹⁴C]PII preparations were equivalent with respect to chromatographic and spectrophotometric characteristics, as well as tumoricidal photodynamic activity in the DBA/2 Ha‐DD mouse: SMT‐F tumor system, to the unlabeled commercial product Photofrin^™ II. The distribution of [¹⁴C]PII in mouse tissues was in close agreement to that previously reported, after adjustment for dose, for [¹⁴C]HPD biosynthetically labeled in vivo (Gomer and Dougherty, 1979), as well as for Photofrin^™ II, where tissue levels were determined spectrophotometrically after extraction (Dougherty and Mang, unpublished).