Abstract 1760: Development of bisphosphonate prodrugs of doxorubicin for targeting bone metastases that are cleaved pH dependently or by cathepsin B

Abstract

Abstract Bone metastases occur in up to 80 % of patients with advanced breast or prostate cancer and significantly affect the quality of life. Most frequently, bone metastases cause bone fractures, severe pain and hypercalcaemia. Current options for treating of bone metastases are surgery, radiotherapy, chemotherapy, hormone therapy, and the administration of bisphosphonates. These treatments are primarily palliative and can improve the quality of life in some cases, however without increasing the survival time of the patients. Bisphosphonates are presently the most active inhibitors of bone degradation, and they can reduce bone pain, bone destruction and tumor growth. Their therapeutic effects are due to the high affinity between bisphosphonates and the apatite structure of bone and bone metastases. In order to improve the outcome of treating bone metastases, new therapeutic approaches are urgently required. As bone-seeking agents, bisphosphonates show an uptake in the bone of 20-60% of the applied dose, and in general the uptake in bone metastases is 10-20-fold higher than in healthy bone tissue. In addition, bone metastases are characterized by an acidic pH and the over-expression of several proteases including cathepsins, matrix metalloproteases, plasmin and uPA. Surprisingly, no prodrug approaches have been reported to date that exploit the bone targeting effect of low-molecular weight bisphosphonates with a concomitant release of a bound drug in bone metastases. To this purpose, we developed water-soluble bisphosphonate prodrugs of doxorubicin for targeting bone metastases by conjugating a thiol-bearing aliphatic bisphosphonate with the 6-maleimidocaproyl hydrazone derivative of doxorubicin (1) that is cleaved pH dependently, or by attaching a maleimide derivative of the dipeptide Val-Ala-PABC (PABC = p-benzyloxycarbonyl) to the aliphatic bisphosphonate (2). Incubation studies with cathepsin B and 2 at 37 °C showed a cleavage of doxorubicin after two hours with a half time of ∼ 45 min and 1 released doxorubicin quantitatively at pH 5.0 after 2 h. The plasma stabilities (human and mouse) of both prodrugs were in the range of 3-6 h at 37 °C. Binding studies of 1 and 2 with a major component of bone, i.e. hydroxyapatite, demonstrated binding of 80-90 % of both prodrugs after 4 h at 37 °C. Finally, in an orientating toxicity study in nude mice the maximum tolerated dose (MTD) of 1 was three-fold higher compared to conventional doxorubicin whereas 2 showed essentially the same MTD as doxorubicin. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1760. doi:1538-7445.AM2012-1760