The synthesis of a prodrug of doxorubicin designed to provide reduced systemic toxicity and greater target efficacy

Doxorubicin (Dox) can provide some stabilization in prostate cancer; howeve r, its use is limited because of systemic toxicities, primarily cardiotoxic ity and immunosuppression. The administration of a prodrug of doxorubicin, designed to permit selective activation by the tumor, would reduce general systemic exposure to the active drug and would thereby increase the therape utic index. Prostate specific antigen (PSA) is a serine protease with chymo trypsin-like activity that is a member of the kallikrein gene family. PSA's putative physiological role is the liquefaction of semen by virtue of its ability to cleave the seminal fluid proteins semenogelins I and II. Serum P SA levels have been found to correlate well with the number of malignant pr ostate cells. The use of a prodrug which is cleaved by the enzyme PSA in th e prostate should in principle produce high localized concentrations of the cytotoxic agent at the tumor site while limiting systemic exposure to the active drug. Cleavage maps following PSA treatment of human semenogelin wer e constructed. Systematic modification of the amino acid residues flanking the primary cleavage site led to the synthesis of a series of short peptide s which were efficiently hydrolyzed by PSA. Subsequent coupling of selected peptides to doxorubicin provided a series of doxorubicin-peptide conjugate s which were evaluated in vitro and in vivo as targeted prodrugs for PSA-se creting tumor cells. From these studies we selected Glutaryl-Hyp-Ala-Ser-Ch g-Gln-Ser-Leu-Dox, 27, as the peptide-doxorubicin conjugate with the best p rofile of physical and biological properties. Compound 27 has a greater tha n 20-fold selectivity against human prostate PSA-secreting LNCaP cells rela tive to the non-PSA-secreting DuPRO cell line. In nude mouse xenograft stud : es, 27 reduced PSA levels by 95% and tumor weight by 87% at a dose below its MTD. Both doxorubicin and Leu-Dox (13) were ineffective in reducing cir culating PSA and tumor burden at their maximum tolerated doses. On the basi s of these results, we selected 27 for further Study to assess its ability to inhibit human prostate cancer cell growth and tumorigenesis.