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Heterobivalent agents targeting PSMA and integrin-αvβ3.

ABSTRACT: Differential expression of surface proteins on normal vs malignant cells provides the rationale for the development of receptor-, antigen-, and transporter-based, cancer-selective imaging and therapeutic agents. However, tumors are heterogeneous, and do not always express what can be considered reliable, tumor-selective markers. That suggests development of more flexible targeting platforms that incorporate multiple moieties enabling concurrent targeting to a variety of putative markers. We report the synthesis, biochemical, in vitro, and preliminary in vivo evaluation of a new heterobivalent (HtBv) imaging agent targeting both the prostate-specific membrane antigen (PSMA) and integrin-αvβ3 surface markers, each of which can be overexpressed in certain tumor epithelium and/or neovasculature. The HtBv agent was functionalized with either 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) or the commercially available IRDye800CW. DOTA-conjugated HtBv probe 9 bound to PSMA or αvβ3 with affinities similar to those of monovalent (Mnv) compounds designed to bind to their targets independently. In situ energy minimization experiments support a model describing the conformations adapted by 9 that enable it to bind both targets. IRDye800-conjugated HtBv probe 10 demonstrated target-specific binding to either PSMA or integrin-αvβ3 overexpressing xenografts. HtBv agents 9 and 10 may enable dual-targeted imaging of malignant cells and tissues in an effort to address heterogeneity that confounds many cancer-targeted imaging agents.

SUBMITTER: Shallal HM

PROVIDER: S-EPMC4112557 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Combining two peptides addressing two different receptors to a heterobivalent peptidic ligand (HBPL) is thought to enable an improved tumor-targeting sensitivity and thus tumor visualization, compared to monovalent peptide ligands. In the case of melanoma, the Melanocortin-1 receptor (MC1R), which is stably overexpressed in the majority of primary malignant melanomas, and integrin αvβ3, which is involved in lymph node metastasis and therefore has an important role in the transition from local to metastatic disease, are important target receptors. Thus, if a radiolabeled HBPL could be developed that was able to bind to both receptor types, the early diagnosis and correct staging of the disease would be significantly increased. Here, we report on the design, synthesis, radiolabeling and in vitro and in vivo testing of different SiFAlin-modified HBPLs (SiFA = silicon fluoride acceptor), consisting of an MC1R-targeting (GG-Nle-c(DHfRWK)) and an integrin αvβ3-affine peptide (c(RGDfK)), being connected by a symmetrically branching framework including linkers of differing length and composition. Kit-like 18F-radiolabeling of the HBPLs 1-6 provided the labeled products [18F]1-[18F]6 in radiochemical yields of 27-50%, radiochemical purities of ≥95% and non-optimized molar activities of 17-51 GBq/μmol within short preparation times of 25 min. Besides the evaluation of radiotracers regarding logD(7.4) and stability in human serum, the receptor affinities of the HBPLs were investigated in vitro on cell lines overexpressing integrin αvβ3 (U87MG cells) or the MC1R (B16F10). Based on these results, the most promising compounds [18F]2, showing the highest affinity to both target receptors (IC50 (B16F10) = 0.99 ± 0.11 nM, IC50 (U87MG) = 1300 ± 288 nM), and [18F]4, exhibiting the highest hydrophilicity (logD(7.4) = -1.39 ± 0.03), were further investigated in vivo and ex vivo in a xenograft mouse model bearing both tumors. For both HBPLs, clear visualization of B16F10, as well as U87MG tumors, was feasible. Blocking studies using the respective monospecific peptides demonstrated both peptide binders of the HBPLs contributing to tumor uptake. Despite the somewhat lower target receptor affinities (IC50 (B16F10) = 6.00 ± 0.47 nM and IC50 (U87MG) = 2034 ± 323 nM) of [18F]4, the tracer showed higher absolute tumor uptakes ([18F]4: 2.58 ± 0.86% ID/g in B16F10 tumors and 3.92 ± 1.31% ID/g in U87MG tumors; [18F]2: 2.32 ± 0.49% ID/g in B16F10 tumors and 2.33 ± 0.46% ID/g in U87MG tumors) as well as higher tumor-to-background ratios than [18F]2. Thus, [18F]4 demonstrates to be a highly potent radiotracer for the sensitive and bispecific imaging of malignant melanoma by PET/CT imaging and impressively illustrates the suitability of the underlying concept to develop heterobivalent integrin αvβ3- and MC1R-bispecific radioligands for the sensitive and specific imaging of malignant melanoma by PET/CT.

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Heterobivalent agents targeting PSMA and integrin-αvβ3.

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