Heme oxygenase (HO) is a key
enzyme in
heme metabolism; it oxidatively degrades
heme to
biliverdin, accompanied by formation of free
iron and
carbon monoxide.
Biliverdin is subsequently reduced by cytosolic
biliverdin reductase to form
bilirubin, a potent
antioxidant. We recently found that
tumor cells utilize HO to protect themselves from oxidative stress by producing the
antioxidant bilirubin. This result suggested an important potential therapeutic strategy: suppression of
bilirubin production with the use of HO inhibitors; hence,
cancer cells become vulnerable to oxidative stress induced by anticancer drugs or leukocytes of the host. This concept was validated by using the intraarterial administration of an HO inhibitor,
zinc protoporphyrin, in nonphysiological
solution. In the present study,
zinc protoporphyrin (ZnPP) was conjugated with poly(
ethylene glycol) (PEG) with molecular weight of 5000, to make ZnPP, a water-soluble compound (
PEG-ZnPP), and to improve its
tumor-targeting efficiency. PEG was conjugated to ZnPP through newly introduced amino groups, where
ethylenediamine residues were added at C6 and C7 of
protoporphyrin. The divalent
zinc cation was chelated into the
protoporphyrin ring to obtain
PEG-ZnPP.
PEG-ZnPP did become highly water-soluble, and it formed multimolecular associations with molecules larger than 70 kDa in aqueous media.
PEG-ZnPP inhibited splenic microsomal HO activity in vitro in a competitive manner in the presence of
hemin, with an apparent inhibitory constant of 0.12 microM. Most important,
PEG-ZnPP injected intravenously significantly suppressed intratumor HO activity in a murine solid
tumor model, which suggests that
tumor-targeted inhibition of HO is possible with the use of
PEG-ZnPP.