99mTc(I)/Re(I) tricarbonyl complexes for in vivo targeting of melanotic melanoma: Synthesis and biological evaluation

Melanin-Targeting Radiotracers and Their Preclinical, Translational, and Clinical Status: From Past to Future

Melanin is one of the representative biomarkers of malignant melanoma and a potential target for diagnosis and therapy. With advancements in chemistry and radiolabeling technologies, promising strides have been made to synthesize radiolabeled melanin-binding molecules for various applications. We present an overview of melanin-targeted radiolabeled molecules and compare their features reported in preclinical studies. Clinical practice and trials are also discussed to elaborate on the safety and validity of the probes, and expanded applications beyond melanoma are reviewed. Melanin-targeted imaging holds potential value in the diagnosis, staging, and prognostic assessment of melanoma and other applications. Melanin-targeted radionuclide therapy possesses immense potential but requires more clinical validation. Furthermore, an intriguing avenue for future research involves expanding the application scope of melanin-targeted probes and exploring their value.

Benzamide derivative radiotracers targeting melanin for melanoma imaging and therapy: Preclinical/clinical development and combination with other treatments

Cutaneous melanoma arises from proliferating melanocytes, cells specialized in the production of melanin. This property means melanin can be considered as a target for monitoring melanoma patients using nuclear imaging or targeted radionuclide therapy (TRT). Since the 1970s, many researchers have shown that specific molecules can interfere with melanin. This paper reviews some such molecules: benzamide structures improved to increase their pharmacokinetics for imaging or TRT. We first describe the characteristics and biosynthesis of melanin, and the main features of melanin tracers. The second part summarizes the preclinical and corresponding clinical studies on imaging. The last section presents TRT results from ongoing protocols and discusses combinations with other therapies as an opportunity for melanoma non-responders or patients resistant to treatments.

Design and Investigation of a [18F]-Labeled Benzamide Derivative as a High Affinity Dual Sigma Receptor Subtype Radioligand for Prostate Tumor Imaging

High overexpression of sigma (σ) receptors (σ₁ and σ₂ subtypes) in a variety of human solid tumors has prompted the development of σ receptor-targeting radioligands, as imaging agents for tumor detection. A majority of these radioligands to date target the σ₂ receptor, a potential marker of tumor proliferative status. The identification of approximately equal proportions of both σ receptor subtypes in prostate tumors suggests that a high affinity, dual σ receptor-targeting radioligand could potentially provide enhanced tumor targeting efficacy in prostate cancer. To accomplish this goal, we designed a series of ligands which bind to both σ receptor subtypes with high affinity. Ligand 3a in this series, displaying optimal dual σ receptor subtype affinity (σ₁, 6.3 nM; σ₂, 10.2 nM) was radiolabeled with fluorine-18 (¹⁸F) to give [¹⁸F]3a and evaluated as a σ receptor-targeting radioligand in the mouse PC-3 prostate tumor model. Cellular assays with PC-3 cells demonstrated that a major proportion of [¹⁸F]3a was localized to cell surface σ receptors, while ∼10% of [¹⁸F]3a was internalized within cells after incubation for 3.5 h. Serial PET imaging in mice bearing PC-3 tumors revealed that uptake of [¹⁸F]3a was 1.6 ± 0.8, 4.4 ± 0.3, and 3.6 ± 0.6% ID/g (% injection dose per gram) in σ receptor-positive prostate tumors at 15 min, 1.5 h, and 3.5 h postinjection, respectively (n = 3) resulting in clear tumor visualization. Blocking studies conducted with haloperidol (a nonselective inhibitor for both σ receptor subtypes) confirmed that the uptake of [¹⁸F]3a was σ receptor-mediated. Histology analysis confirmed similar expression of σ₁ and σ₂ in PC-3 tumors which was significantly greater than its expression in normal organs/tissues such as liver, kidney, and muscle. Metabolite studies revealed that >50% of radioactivity in PC-3 tumors at 30 min postinjection represented intact [¹⁸F]3a. Prominent σ receptor-specific uptake of [¹⁸F]3a in prostate tumors and its subsequent clear visualization with PET imaging indicate potential utility for the diagnosis of prostate carcinoma.

Isostructural Re(i)/99mTc(i) tricarbonyl complexes for cancer theranostics

Novel cysteamine-based (N,S,O)-chelators were successfully applied in the synthesis of isostructural M(i) (M = Re, ^99mTc) tricarbonyl complexes for cancer theranostics.

Clickable, hydrophilic ligand for fac-[M(I)(CO)3](+) (M = Re/(99m)Tc) applied in an S-functionalized α-MSH peptide

The copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) click reaction was used to incorporate alkyne-functionalized dipicolylamine (DPA) ligands (1 and 3) for fac-[M^I(CO)₃]⁺ (M = Re/^99mTc) complexation into an α-melanocyte stimulating hormone (α-MSH) peptide analogue. A novel DPA ligand with carboxylate substitutions on the pyridyl rings (3) was designed to increase the hydrophilicity and to decrease in vivo hepatobiliary retention of fac-[^99mTc^I(CO)₃]⁺ complexes used in single photon emission computed tomography (SPECT) imaging studies with targeting biomolecules. The fac-[Re^I(CO)₃(3)] complex (4) was used for chemical characterization and X-ray crystal analysis prior to radiolabeling studies between 3 and fac-[^99mTc^I(OH₂)₃(CO)₃]⁺. The corresponding ^99mTc complex (4a) was obtained in high radiochemical yields, was stable in vitro for 24 h during amino acid challenge and serum stability assays, and showed increased hydrophilicity by log P analysis compared to an analogous complex with nonfunctionalized pyridine rings (2a). An α-MSH peptide functionalized with an azide was labeled with fac-[M^I(CO)₃]⁺ using both click, then chelate (CuAAC reaction with 1 or 3 followed by metal complexation) and chelate, then click (metal complexation of 1 and 3 followed by CuAAC with the peptide) strategies to assess the effects of CuAAC conditions on fac-[M^I(CO)₃]⁺ complexation within a peptide framework. The peptides from the click, then chelate strategy had different HPLC t_R’s and in vitro stabilities compared to those from the chelate, then click strategy, suggesting nonspecific coordination of fac-[M^I(CO)₃]⁺ using this synthetic route. The fac-[M^I(CO)₃]⁺-complexed peptides from the chelate, then click strategy showed >90% stability during in vitro challenge conditions for 6 h, demonstrated high affinity and specificity for the melanocortin 1 receptor (MC1R) in IC₅₀ analyses, and led to moderately high uptake in B16F10 melanoma cells. Log P analysis of the ^99mTc-labeled peptides confirmed the enhanced hydrophilicity of the peptide bearing the novel, carboxylate-functionalized DPA chelate (10a′) compared to the peptide with the unmodified DPA chelate (9a′). In vivo biodistribution analysis of 9a′ and 10a′ showed moderate tumor uptake in a B16F10 melanoma xenograft mouse model with enhanced renal uptake and surprising intestinal uptake for 10a′ compared to predominantly hepatic accumulation for 9a′. These results, coupled with the versatility of CuAAC, suggests this novel, hydrophilic chelate can be incorporated into numerous biomolecules containing azides for generating targeted fac-[M^I(CO)₃]⁺ complexes in future studies.

Novel, cysteine-modified chelation strategy for the incorporation of [M(I)(CO)(3)](+) (M = Re, (99m)Tc) in an α-MSH peptide

Engineering peptide-based targeting agents with residues for site-specific and stable complexation of radionuclides is a highly desirable strategy for producing diagnostic and therapeutic agents for cancer and other diseases. In this report, a model N-S-N(Py) ligand (3) and a cysteine-derived α-melanocyte stimulating hormone (α-MSH) peptide (6) were used as novel demonstrations of a widely applicable chelation strategy for incorporation of the [M(I)(CO)(3)](+) (M = Re, (99m)Tc) core into peptide-based molecules for radiopharmaceutical applications. The structural details of the core ligand-metal complexes as model systems were demonstrated by full chemical characterization of fac-[Re(I)(CO)(3)(N,S,N(Py)-3)](+) (4) and comparative high-performance liquid chromatography (HPLC) analysis between 4 and [(99m)Tc(I)(CO)(3)(N,S,N(Py)-3)](+) (4a). The α-MSH analogue bearing the N-S-N(Py) chelate on a modified cysteine residue (6) was generated and complexed with [M(I)(CO)(3)](+) to confirm the chelation strategy's utility when applied in a peptide-based targeting agent. Characterization of the Re(I)(CO)(3)-6 peptide conjugate (7) confirmed the efficient incorporation of the metal center, and the (99m)Tc(I)(CO)(3)-6 analogue (7a) was explored as a potential single photon emission computed tomography (SPECT) compound for imaging the melanocortin 1 receptor (MC1R) in melanoma. Peptide 7a showed excellent radiolabeling yields and in vitro stability during amino acid challenge and serum stability assays. In vitro B16F10 melanoma cell uptake of 7a reached a modest value of 2.3 ± 0.08% of applied activity at 2 h at 37 °C, while this uptake was significantly reduced by coincubation with a nonlabeled α-MSH analogue, NAPamide (3.2 μM) (P < 0.05). In vivo SPECT/X-ray computed tomography (SPECT/CT) imaging and biodistribution of 7a were evaluated in a B16F10 melanoma xenografted mouse model. SPECT/CT imaging clearly visualized the tumor at 1 h post injection (p.i.) with high tumor-to-background contrast. Blocking studies with coinjected NAPamide (10 mg per kg of mouse body weight) confirmed the in vivo specificity of 7a for MC1R-positive tumors. Biodistribution results with 7a yielded a moderate tumor uptake of 1.20 ± 0.09 percentage of the injected radioactive dose per gram of tissue (% ID/g) at 1 h p.i. Relatively high uptake of 7a was also seen in the kidneys and liver at 1 h p.i. (6.55 ± 0.36% ID/g and 4.44 ± 0.17% ID/g, respectively), although reduced kidney uptake was seen at 4 h p.i. (3.20 ± 0.48% ID/g). These results demonstrate the utility of the novel [M(I)(CO)(3)](+) chelation strategy when applied in a targeting peptide.