Development of a Novel Imaging Agent for Determining Albumin Uptake in Solid Tumors.

PURPOSE: The purpose of this study was to investigate the albumin-binding compound 111In-C4-DTPA as an imaging agent for the detection of endogenous albumin accumulation in tumors. METHODS: 111In-C4-DTPA was injected in healthy nude mice for pharmacokinetic and biodistribution studies (10 min, 1, 6, 24, and 48 h, n = 4) and subsequently in tumor-bearing mice for single-photon emission computed tomography/X-ray-computed tomography (SPECT/CT) imaging studies. Four different human tumor xenograft models (LXFL529, OVXF899, MAXFTN401, and CXF2081) were implanted subcutaneously unilaterally or bilaterally (n = 4-8). After intravenous administration of 111In-C4-DTPA, SPECT/CT images were collected over 72 h at 4-6 time points. Additionally, gamma counting was performed for the blood, plasma, lungs, heart, liver, spleen, kidneys, muscle, and tumors at 72 h post-injection. RESULTS: 111In-C4-DTPA bound rapidly to circulating albumin upon injection, and the radiolabeled albumin conjugate thus formed was stable in murine and human serum. SPECT/CT images demonstrated a time-dependent uptake with a maximum of 2.7-3.8% ID/cm3 in the tumors at approximately 24 h post-injection and mean tumor/muscle ratios in the range of 3.2-6.2 between 24 and 72 h post-injection. The kidneys and bladder were the predominant elimination organs. Gamma counting at 72 h post-injection showed 1.3-2.5% ID/g in the tumors and mean tumor/muscle ratios in the range of 4.9-9.4. CONCLUSION: 111In-C4-DTPA bound rapidly to circulating albumin upon injection and showed time-dependent uptake in the tumors demonstrating a potential for clinical application as a companion imaging diagnostic for albumin-binding anticancer drugs.

In vivo evaluation of a novel albumin-binding prodrug of doxorubicin in an orthotopic mouse model of prostate cancer (LNCaP).

PSA, which is overexpressed in prostate carcinoma, represents a molecular target for selectively releasing an anticancer agent from a prodrug formulation. In this study, we report on the in vivo antitumor efficacy of an efficacious albumin-binding prodrug of doxorubicin (PSA9) that incorporates p-aminobenzyloxycarbonyl (PABC) as a self-immolative spacer in addition to the heptapeptide, Arg-Ser-Ser-Tyr-Tyr-Ser-Leu, which serves as a substrate for PSA. The prodrug is cleaved very efficiently by PSA releasing H-Ser-Leu-PABC-doxorubicin and subsequently doxorubicin in PSA-positive cell lysates and prostate tumor homogenates as the final cleavage product. PSA9 at 3 × 6 mg kg(-1) doxorubicin equivalents (intravenous) was compared with conventional doxorubicin at equitoxic doses (at 3 × 3 mg kg(-1); intravenous) in an orthotopic mouse model of prostate cancer using LNCaP lentiviral luciferase-neomycin cells transduced with luciferase. Whereas doxorubicin did not show any efficacy against the primary tumor or metastases, the prodrug reduced the primary tumor by 30-50% and circulating PSA levels, and in addition, showed a pronounced reduction in lung and bone metastases by ∼77% and ∼96%, respectively, and a positive trend regarding the activity against liver and lymph-node metastases compared with control and doxorubicin-treated animals. The incorporation of PABC as a self-immolative spacer together with a PSA substrate demonstrates superior antitumor effects over doxorubicin attributed to an efficient cleavage by PSA releasing doxorubicin as the final active agent in prostate tumor homogenates. Using this approach for developing effective prodrugs against prostate cancer, is worthy of further preclinical optimization.