Gallium-68-labeled DOTA-rhenium-cyclized α-melanocyte-stimulating hormone analog for imaging of malignant melanoma

Abstract

Introduction

Diagnosis of malignant melanoma is critical, since a patient's prognosis is poor. Previous studies have shown that ⁶⁴Cu- and ⁸⁶Y-DOTA-ReCCMSH(Arg¹¹) have the potential for early detection of malignant melanoma by exploiting the sensitivity and high resolution of positron emission tomography (PET). This encouraged us to investigate DOTA-ReCCMSH(Arg¹¹) labeled with another β⁺-emitting radionuclide, ⁶⁸Ga.

Methods

DOTA-ReCCMSH(Arg¹¹) was successfully labeled with ⁶⁸Ga at pH 3.8–4 at 85°C. Acute biodistribution and small-animal PET imaging studies were performed in mice bearing B16/F1 melanoma tumor.

Results

Biodistribution studies showed moderate receptor-mediated tumor uptake, fast nontarget organ clearance and high tumor to nontarget tissue ratios. Preadministration of d-lysine significantly reduced kidney uptake without affecting the uptake of the agent in the tumor. Small-animal PET images showed that the tumor could be clearly visualized at all time points examined (0.5–2 h) with the standardized uptake value analysis following a similar trend as the biodistribution data.

Conclusions

The preliminary data obtained suggest that ⁶⁸Ga-DOTA-ReCCMSH(Arg¹¹) is a promising PET imaging agent for early detection of malignant melanoma.

Introduction

Malignant melanoma, the most serious form of skin cancer, has become a severe health problem due to an increase in incidence and mortality rate [1], [2]. Accurate diagnosis is critical, because unless the primary tumor is excised through surgery, patient prognosis is generally poor [3]. 2-[¹⁸F]fluoro-2-deoxy-d-glucose ([¹⁸F]FDG) [4], [5], [6] and melanoma-targeting antibodies or antibody fragments [7], [8], [9] have been developed for imaging and staging of disease. [¹⁸F]FDG positron emission tomography (PET) is limited in detecting melanoma tumors with small foci [5], and some melanoma cells are undetectable with [¹⁸F]FDG because they use non-glucose-based substrates as an energy source [6]; therefore, their clinical application has been limited. Radiolabeled antibodies generally lack specificity due to the presence of individual tumor variants, and since melanomas tend to become amelanotic as the malignancy progresses, the target antigen is often lost due to the reduction in expression of the specific pigment genes that encode this target [10], [11], [12]. The increasing numbers of amelanotic cells in primary tumors or their metastasis increases the possibility that such cells will escape targeting by antibody-based radiopharmaceuticals. Examples of other agents that have been examined clinically for the imaging of melanoma, as alternatives to [¹⁸F]FDG, include the PET agents [¹⁸F]galacto-RGD [13], 6-[¹⁸F]fluoro-l-dopa ([¹⁸F]DOPA) [6] and 3′-[¹⁸F]fluoro-3′-deoxy-l-thymidine ([¹⁸F]FLT) [14] and the SPECT agents N-isopropyl-p-¹²³I-iodoamphetamine ([¹²³I]IMP) [15], ¹²³I-N-(2-diethylaminoethyl)-2-iodobenzamide ([¹²³I]BZA(2)) [16] and ¹²³I-N-[3-(4-morpholino)propyl]-N-methyl-2-hydroxy-5-iodo-3-methylbenzylamine (ERC9) [17].

Recently, alpha-melanocyte-stimulating hormone (α-MSH) peptide analogs were developed as promising agents for both melanoma imaging and radiotherapy. α-MSH is a tridecapeptide that is excreted by the pars intermedia of the pituitary gland and is primarily responsible for the regulation of skin pigmentation [18], [19]. α-MSH targets the melanocortin-1 receptor (MC1R), which belongs to a family of G-protein-coupled receptors (MC1R to MC5R) that have been isolated in both mice and humans [20], [21], [22], [23], [24], [25], [26]. These receptors are normally expressed on the surface of melanocytes, and it was reported that >80% of human melanoma tumor samples obtained from patients with metastatic lesions express α-MSH receptors [27], [28]. It has been reported the MC1R expression ranges from several hundred to around 10,000 receptors per cell [29]. In 2003, Miao et al. [30] reported that MC1R receptor expression ranged from 900 (1.49 fmol per million SKMEL28 cells) to around 6000 (9.46 fmol per million TXM13 cells) receptors per cell on human melanoma cells, enabling the use of radiolabeled α-MSH peptide analogs as specific melanoma diagnostic and therapeutic agents.

There have been a number of reports on α-MSH peptide-based agents that have demonstrated the ability to image the MCR1 receptor in vivo [31], [32], [33], [34], [35], [36], [37], [38], [39]. We previously described an α-MSH-targeting peptide system, DOTA-ReCCMSH(Arg¹¹), which was labeled with β⁺-emitting radiometals, as potential agents for the detection of malignant melanoma via PET imaging [40]. The rhenium-cyclized peptide system was chosen for further study given its increased stability compared to its linear analog with a concomitant increase in tumor uptake and less renal retention [38], [39]. We chose the divalent metal ion ⁶⁴Cu (t_1/2=12.7 h, 17.4% β⁺, 41% electron capture (EC), 40% β⁻) [41] and trivalent metal ion ⁸⁶Y (t_1/2=14.7 h, 33% β⁺), both of which can be prepared on a biomedical cyclotron utilizing the ⁶⁴Ni(p,n)⁶⁴Cu and ⁸⁶Sr(p,n)⁸⁶Y reactions, respectively [42], [43]. Our results showed that both ⁶⁴Cu-DOTA-ReCCMSH(Arg¹¹) and ⁸⁶Y-DOTA-ReCCMSH(Arg¹¹) have the potential for early detection of malignant melanoma by exploiting the sensitivity and high resolution of PET [40]. This encouraging result motivated us to investigate the same peptide DOTA-ReCCMSH(Arg¹¹) radiolabeled with another β⁺-emitting radiometal ⁶⁸Ga. ⁶⁸Ga (t_1/2=68 min, 88% β⁺) is a short-lived positron emitter, the production of which is not dependent on an in-house cyclotron since it is available from commercially available ⁶⁸Ge/⁶⁸Ga generators that have >1-year shelf lives. The use of ⁶⁸Ga-labeled peptides is an emerging area of PET radiopharmaceutical development and a number of agents have been translated to the clinic [44]. This report describes our preliminary study on ⁶⁸Ga labeled DOTA-ReCCMSH(Arg¹¹), and, to the best of our knowledge, this is the first study on ⁶⁸Ga-labeled DOTA-conjugated cyclized α-MSH peptide analog. We report small-animal PET and biodistribution studies of ⁶⁸Ga-DOTA-ReCCMSH(Arg¹¹) in the B16/F1 tumor-bearing mouse model.