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; however, its use is limited because of systemic toxicities, primarily cardiotoxicity 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 therapeutic index. Prostate specific antigen (PSA) is a serine protease with chymotrypsin-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 PSA levels have been found to correlate well with the number of malignant prostate cells. The use of a prodrug which is cleaved by the enzyme PSA in the 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 were constructed. Systematic modification of the amino acid residues flanking the primary cleavage site led to the synthesis of a series of short peptides which were efficiently hydrolyzed by PSA. Subsequent coupling of selected peptides to doxorubicin provided a series of doxorubicin-peptide conjugates which were evaluated in vitro and in vivo as targeted prodrugs for PSA-secreting tumor cells. From these studies we selected Glutaryl-Hyp-Ala-Ser-Chg-Gln-Ser-Leu-Dox, <BO>27</BO>, as the peptide-doxorubicin conjugate with the best profile of physical and biological properties. Compound <BO>27</BO> has a greater than 20-fold selectivity against human prostate PSA-secreting LNCaP cells relative to the non-PSA-secreting DuPRO cell line. In nude mouse xenograft studies, <BO>27</BO> reduced PSA levels by 95% and tumor weight by 87% at a dose below its MTD. Both doxorubicin and Leu-Dox (<BO>13</BO>) were ineffective in reducing circulating PSA and tumor burden at their maximum tolerated doses. On the basis of these results, we selected <BO>27</BO> for further study to assess its ability to inhibit human prostate cancer cell growth and tumorigenesis.