Protoporphyrin dimethyl ester (I) is treated with HBr in HOAc to produce dibromide (II). Displacement of the bromide groups of (II) by poly(ethyleneglycol)-750-mono-methyl ether, followed by alkaline hydrolysis of the concomitantly transesterified ester groups, furnishes the porphyrin di-ether (III). This is finally complexed with diammine(diaqua)platinum(II) hydroxide to yield the title compound.