Radiolabeled Melanin-Binding Peptides Are Safe and Effective in Treatment of Human Pigmented Melanoma in a Mouse Model of Disease

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.

Bifunctional chelators for radiorhenium: past, present and future outlook

Targeted radionuclide therapy (TRNT) is an ever-expanding field of nuclear medicine that provides a personalised approach to cancer treatment while limiting toxicity to normal tissues. It involves the radiolabelling of a biological targeting vector with an appropriate therapeutic radionuclide, often facilitated by the use of a bifunctional chelator (BFC) to stably link the two entities. The radioisotopes of rhenium, 186Re (t 1/2 = 90 h, 1.07 MeV β-, 137 keV γ (9%)) and 188Re (t 1/2 = 16.9 h, 2.12 MeV β-, 155 keV γ (15%)), are particularly attractive for radiotherapy because of their convenient and high-abundance β--particle emissions as well as their imageable γ-emissions and chemical similarity to technetium. As a transition metal element with multiple oxidation states and coordination numbers accessible for complexation, there is great opportunity available when it comes to developing novel BFCs for rhenium. The purpose of this review is to provide a recap on some of the past successes and failings, as well as show some more current efforts in the design of BFCs for 186/188Re. Future use of these radionuclides for radiotherapy depends on their cost-effective availability and this will also be discussed. Finally, bioconjugation strategies for radiolabelling biomolecules with 186/188Re will be touched upon.

SHIRPA as a Neurological Screening Battery in Mice

The SmithKline, Harwell, Imperial College, Royal Hospital, Phenotype Assessment (SHIRPA) is a rapid battery of tests comprising 42 measurements of motor activity, coordination, postural control, muscle tone, autonomic functions, and emotional reactivity, as well as reflexes dependent on visual, auditory, and tactile modalities. Individual scores in SHIRPA are sensitive in detecting phenotypes of several experimental models of neural disease, especially cerebellar degeneration and Alzheimer disease, and combined subscores have been useful in estimating the impact of vascular anomalies and exposure to infectious agents. In cerebellar degeneration, weak forelimb grip, impaired wire maneuver and air righting, and negative geotaxis appear as prevalent features. Most of the measures in the battery are susceptible to change after gene modifications or physiological alterations. SHIRPA can be used both in adult mice and mice in the preweaning period to screen for sensorimotor function and emotional reactivity, not selective attention or memory. © 2021 Wiley Periodicals LLC Basic Protocol: Step-by-step procedure for SHIRPA.

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.

Targeted melanoma radiotherapy using ultrasmall 177Lu-labeled α-melanocyte stimulating hormone-functionalized core-shell silica nanoparticles

Lutetium-177 (¹⁷⁷Lu) radiolabeled ultrasmall (~6 nm dia.) fluorescent core-shell silica nanoparticles (Cornell prime dots or C' dots) were developed for improving efficacy of targeted radiotherapy in melanoma models. PEGylated C' dots were surface engineered to display 10-15 alpha melanocyte stimulating hormone (αMSH) cyclic peptide analogs for targeting the melanocortin-1 receptor (MC1-R) over-expressed on melanoma tumor cells. The ¹⁷⁷Lu-DOTA-αMSH-PEG-C' dot product was radiochemically stable, biologically active, and exhibited high affinity cellular binding properties and internalization. Selective tumor uptake and favorable biodistribution properties were also demonstrated, in addition to bulk renal clearance, in syngeneic B16F10 and human M21 xenografted models. Prolonged survival was observed in the treated cohorts relative to controls. Dosimetric analysis showed no excessively high absorbed dose among normal organs. Correlative histopathology of ex vivo treated tumor specimens revealed expected necrotic changes; no acute pathologic findings were noted in the liver or kidneys. Collectively, these results demonstrated that ¹⁷⁷Lu-DOTA-αMSH-PEG-C' dot targeted melanoma therapy overcame the unfavorable biological properties and dose-limiting toxicities associated with existing mono-molecular treatments. The unique and tunable surface chemistries of this targeted ultrasmall radiotherapeutic, coupled with its favorable pharmacokinetic properties, substantially improved treatment efficacy and demonstrated a clear survival benefit in melanoma models, which supports its further clinical translation.

Nanomelanin Potentially Protects the Spleen from Radiotherapy-Associated Damage and Enhances Immunoactivity in Tumor-Bearing Mice

Radiotherapy side-effects present serious problems in cancer treatment. Melanin, a natural polymer with low toxicity, is considered as a potential radio-protector; however, its application as an agent against irradiation during cancer treatment has still received little attention. In this study, nanomelanin particles were prepared, characterized and applied in protecting the spleens of tumor-bearing mice irradiated with X-rays. These nanoparticles had sizes varying in the range of 80–200 nm and contained several important functional groups such as carboxyl (-COO), carbonyl (-C=O) and hydroxyl (-OH) groups on the surfaces. Tumor-bearing mice were treated with nanomelanin at a concentration of 40 mg/kg before irradiating with a single dose of 6.0 Gray of X-ray at a high dose rate (1.0 Gray/min). Impressively, X-ray caused mild splenic fibrosis in 40% of nanomelanin-protected mice, whereas severe fibrosis was observed in 100% of mice treated with X-ray alone. Treatment with nanomelanin also partly rescued the volume and weight of mouse spleens from irradiation through promoting the transcription levels of splenic Interleukin-2 (IL-2) and Tumor Necrosis Factor alpha (TNF-α). More interestingly, splenic T cell and dendritic cell populations were 1.91 and 1.64-fold higher in nanomelanin-treated mice than those in mice which received X-ray alone. Consistently, the percentage of lymphocytes was also significantly greater in blood from nanomelanin-treated mice. In addition, nanomelanin might indirectly induce apoptosis in tumor tissues via activation of TNF-α, Bax, and Caspase-3 genes. In summary, our results demonstrate that nanomelanin protects spleens from X-ray irradiation and consequently enhances immunoactivity in tumor-bearing mice; therefore, we present nanomelanin as a potential protector against damage from radiotherapy in cancer treatment.

Effective melanin degradation by a synergistic laccase-peroxidase enzyme complex for skin whitening and other practical applications

Melanin is major cause of dark skin, which is regarded as social status in eastern Asia. As a result, researchers in cosmetic industries are developing skin whitening agents. Melanin can be decolorized by many oxidative enzymes. Laccase (CueO) from Escherichia coli and dye-decolorizing peroxidase (DyP) from Bacillus subtilis were merged with the dockerin domain of endoglucanase B from Clostridium cellulovorans. Scaffoldin has great potential to exert structural benefits that enable complementary enzyme effects. The carbohydrate binding module (CBM) in scaffoldin was replaced with the melanin binding peptide (MBP) to increase melanin binding and thereby enhance melanin degradation. The modified scaffoldin exhibits a nearly 64% increase in specific binding to melanin over that of the native scaffoldin. Laccase was used to degrade melanin via the production of hydrogen peroxide, which produced synergistic activity with peroxidase. The activity of the optimized complex was approximately 6.4-fold greater than that of laccase alone. This enzyme complex can also reduce the number of melanin granules in corneocytes. Based on these results, a recombinant enzyme complex is suitable for use in melanin degradation by next generation whitening agents in the skin cosmetics industry.

Molecular Imaging and Radionuclide Therapy of Melanoma Targeting the Melanocortin 1 Receptor

Melanoma is a deadly disease at late metastatic stage, and early diagnosis and accurate staging remain the key aspects for managing melanoma. The melanocortin 1 receptor (MC1 R) is overexpressed in primary and metastatic melanomas, and its endogenous ligand, the α-melanocyte-stimulating hormone (αMSH), has been extensively studied for the development of MC1 R-targeted molecular imaging and therapy of melanoma. Natural αMSH is not well suited for this purpose due to low stability in vivo. Unnatural amino acid substitutions substantially stabilized the peptide, while cyclization via lactam bridge and metal coordination further improved binding affinity and stability. In this study, we summarized the development and the in vitro and in vivo characteristics of the radiolabeled αMSH analogues, including ^99mTc-, ¹¹¹In-, ⁶⁷ Ga-, or ¹²⁵I-labeled αMSH analogues for imaging with single-photon emission computed tomography; ⁶⁸Ga-, ⁶⁴Cu-, or ¹⁸F-labeled αMSH analogues for imaging with positron emission tomography; and ¹⁸⁸Re-, ¹⁷⁷Lu-, ⁹⁰Y-, or ²¹²Pb-labeled αMSH analogues for radionuclide therapy. These radiolabeled αMSH analogues showed promising results with high tumor uptake and rapid normal tissue activity clearance in the preclinical model of B16F1 and B16F10 mouse melanomas. These results highlight the potential of using radiolabeled αMSH analogues in clinical applications for molecular imaging and radionuclide therapy of melanoma.

PET and SPECT imaging of melanoma: the state of the art

Melanoma represents the most aggressive form of skin cancer, and its incidence continues to rise worldwide. 18F-FDG PET imaging has transformed diagnostic nuclear medicine and has become an essential component in the management of melanoma, but still has its drawbacks. With the rapid growth in the field of nuclear medicine and molecular imaging, a variety of promising probes that enable early diagnosis and detection of melanoma have been developed. The substantial preclinical success of melanin- and peptide-based probes has recently resulted in the translation of several radiotracers to clinical settings for noninvasive imaging and treatment of melanoma in humans. In this review, we focus on the latest developments in radiolabeled molecular imaging probes for melanoma in preclinical and clinical settings, and discuss the challenges and opportunities for future development.

Targeted Radionuclide Therapy of Melanoma

An estimated 60,000 individuals in the United States and 132,000 worldwide are yearly diagnosed with melanoma. Until recently, treatment options for patients with stages III-IV metastatic disease were limited and offered marginal, if any, improvement in overall survival. The situation changed with the introduction of B-RAF inhibitors and anti-cytotoxic T-lymphocyte antigen 4 and anti-programmed cell death protein 1 immunotherapies into the clinical practice. With only some patients responding well to the immune therapies and with very serious side effects and high costs of immunotherapy, there is still room for other approaches for the treatment of metastatic melanoma. Targeted radionuclide therapy of melanoma could be divided into the domains of radioimmunotherapy (RIT), radiolabeled peptides, and radiolabeled small molecules. RIT of melanoma is currently experiencing a renaissance with the clinical trials of alpha-emitter (213)Bi-labeled and beta-emitter (188)Rhenium-labeled monoclonal antibodies in patients with metastatic melanoma producing encouraging results. The investigation of the mechanism of efficacy of melanoma RIT points at killing of melanoma stem cells by RIT and involvement of immune system such as complement-dependent cytotoxicity. The domain of radiolabeled peptides for targeted melanoma therapy has been preclinical so far, with work concentrated on radiolabeled peptide analogues of melanocyte-stimulating hormone receptor and on melanin-binding peptides. The field of radiolabeled small molecule produced radioiodinated benzamides that cross the cellular membrane and bind to the intracellular melanin. The recent clinical trial demonstrated measurable antitumor effects and no acute or midterm toxicities. We are hopeful that the targeted radionuclide therapy of metastatic melanoma would become a clinical reality as a stand-alone therapy or in combination with the immunotherapies such as anti-PD1 programmed cell death protein 1 monoclonal antibodies within the next few years.

Behavioral characterization of CD36 knockout mice with SHIRPA primary screen

CD36 is a member of the class B scavenger receptor family of cell surface proteins, which plays a major role in fatty acid, glucose and lipid metabolism. Besides, CD36 functions as a microglial surface receptor for amyloid beta peptide. Regarding this, we suggest CD36 might also contribute to neuropsychiatric disease. The aim of this study was to achieve a behavioral phenotype of CD36 knockout (CD36(-/-)) mice. We characterized the behavior of CD36(-/-) mice and C57BL/6J mice by subjecting them to a series of tests, which include SHIRPA primary behavioral screen test, 1% sucrose preference test, elevated plus-maze test, open-field test and forced swimming test. The results showed that CD36(-/-) mice traversed more squares, emitted more defecation, exhibited higher tail elevation and had more aggressive behaviors than C57BL/6J mice. The CD36(-/-) mice spent more time and traveled longer distance in periphery zone in the open-field test. Meanwhile, the numbers that CD36(-/-) mice entered in the open arms of elevated plus-maze were reduced. These findings suggest that CD36(-/-) mice present an anxious phenotype and might be involved in neuropsychiatric disorders.

Imaging malignant melanoma with (18)F-5-FPN

PURPOSE

Radiolabelled benzamides are attractive candidates for targeting melanoma because they bind to melanin and exhibit high tumour uptake and retention. (18)F-5-Fluoro-N-(2-[diethylamino]ethyl)picolinamide ((18)F-5-FPN), a benzamide analogue, was prepared and its pharmacokinetics and binding affinity evaluated both in vitro and in vivo to assess its clinical potential in the diagnosis and staging of melanoma.

METHODS

(18)F-5-FPN was prepared and purified. Its binding specificity was measured in vitro in two different melanoma cell lines, one pigmented (B16F10 cells) and one nonpigmented (A375m cells), and in vivo in mice xenografted with the same cell lines. Dynamic and static PET images using (18)F-5-FPN were obtained in the tumour-bearing mice, and the static images were also compared with those acquired with (18)F-FDG. PET imaging with (18)F-5-FPN was also performed in B16F10 tumour-bearing mice with lung metastases.

RESULTS

(18)F-5-FPN was successfully prepared with radiochemical yields of 5 - 10 %. Binding of (18)F-5-FPN to B16F10 cells was much higher than to A375m cells. On dynamic PET imaging B16F10 tumours were visible about 1 min after injection of the tracer, and the uptake gradually increased over time. (18)F-5-FPN was rapidly excreted via the kidneys. B16F10 tumours were clearly visible on static images acquired 1 and 2 h after injection, with high uptake values of 24.34 ± 6.32 %ID/g and 16.63 ± 5.41 %ID/g, respectively, in the biodistribution study (five mice). However, there was no visible uptake by A375m tumours. (18)F-5-FPN and (18)F-FDG PET imaging were compared in B16F10 tumour xenografts, and the tumour-to-background ratio of (18)F-5-FPN was ten times higher than that of (18)F-FDG (35.22 ± 7.02 vs. 3.29 ± 0.53, five mice). (18)F-5-FPN PET imaging also detected simulated lung metastases measuring 1 - 2 mm.

CONCLUSION

(18)F-5-FPN specifically targeted melanin in vitro and in vivo with high retention and affinity and favourable pharmacokinetics. (18)F-5-FPN may be an ideal molecular probe for melanoma diagnosis and staging.

Tyrosinase as a multifunctional reporter gene for Photoacoustic/MRI/PET triple modality molecular imaging

Development of reporter genes for multimodality molecular imaging is highly important. In contrast to the conventional strategies which have focused on fusing several reporter genes together to serve as multimodal reporters, human tyrosinase (TYR)--the key enzyme in melanin production--was evaluated in this study as a stand-alone reporter gene for in vitro and in vivo photoacoustic imaging (PAI), magnetic resonance imaging (MRI) and positron emission tomography (PET). Human breast cancer cells MCF-7 transfected with a plasmid that encodes TYR (named as MCF-7-TYR) and non-transfected MCF-7 cells were used as positive and negative controls, respectively. Melanin targeted N-(2-(diethylamino)ethyl)-18F-5-fluoropicolinamide was used as a PET reporter probe. In vivo PAI/MRI/PET imaging studies showed that MCF-7-TYR tumors achieved significant higher signals and tumor-to-background contrasts than those of MCF-7 tumor. Our study demonstrates that TYR gene can be utilized as a multifunctional reporter gene for PAI/MRI/PET both in vitro and in vivo.

Synthesis and Biological Evaluation of New Quinoxaline Derivatives of ICF01012 as Melanoma-Targeting Probes

The aim of this study was the synthesis and pharmacokinetic selection of a best melanin-targeting ligand for addressing anticancer agents to pigmented melanoma. Seven quinoxaline carboxamide derivatives were synthesized and radiolabeled with iodine-125. Biodistribution studies of compounds [ (125) I]1a-g performed in melanoma-bearing mice tumor showed significant tumor uptake (range 2.43-5.68%ID/g) within 1 h after i.v. injection. Fast clearance of the radioactivity from the nontarget organs mainly via the urinary system gave high tumor-to-blood and tumor-to-muscle ratios. Given its favorable clearance and high tumor-melanoma uptake at 72 h, amide 1d was the most promising melanoma-targeting ligand in this series. Compound 1d will be used as building block for the design of new melanoma-selective drug delivery systems.

In vitro and in vivo evaluation of melanin-binding decapeptide 4B4 radiolabeled with 177Lu, 166Ho, and 153Sm radiolanthanides for the purpose of targeted radionuclide therapy of melanoma

Melanoma is a malignancy with increasing incidence. Although primary tumors that are localized to the skin can be successfully treated by surgical removal, there is no satisfactory treatment for metastatic melanoma, a condition that has currently an estimated 5-year survival of just 6%. During the last decade, β- or α-emitter-radiolabeled peptides that bind to different receptors on a variety of tumors have been investigated as potential therapeutic agents in both the preclinical and clinical settings with encouraging results. A recent study demonstrated that 188-Rhenium ((188)Re)-labeled, via HYNIC ligand, fungal melanin-binding decapeptide 4B4 was effective against experimental MNT1 human melanoma and was safe to normal melanized tissues. The availability of radiolanthanides with diverse nuclear emission schemes and half-lives provides an opportunity to expand the repertoire of peptides for radionuclide therapy of melanoma. The melanin-binding decapeptide 4B4 was radiolabeled with (177)Lu, (166)Ho, and (153)Sm via a DO3A chelate. The stability studies of Ln*-DO3A-4B4 in phosphate-buffered saline, serum, and a hydroxyapatite assay demonstrated that (177)Lu-labeled peptide was more stable than (166)Ho- and (153)Sm-labeled peptides, most likely because of the smallest ionic radius of the former allowing for better complexation with DO3A. Binding of Ln*-DO3A-4B4 to the lysed highly melanized MNT1 melanoma cells demonstrated the specificity of peptides binding to melanin. In vivo biodistribution data for (177)Lu-DO3A-4B4 given by intraperitoneal administration to lightly pigmented human metastatic A2058 melanoma-bearing mice demonstrated very high uptake in the kidneys and low tumor uptake. Intravenous administration did not improve the tumor uptake. The plausible explanation of low tumor uptake of (177)Lu-DO3A-4B4 could be its decreased ability to bind to melanin during in vitro binding studies in comparison with (188)Re-HYNIC-4B4, exacerbated by the very fast clearance from the blood and the kidneys "sink" effect.

A preclinical 188Re tumor therapeutic investigation using MORF/cMORF pretargeting and an antiTAG-72 antibody CC49

The utility of MORF/cMORF pretargeting for the radiotherapy of cancer requires further validation in tumored mice before clinical trials. We now report on a therapeutic study in mice pretargeted with MORF-CC49 (an anti-TAG-72 antibody CC49 conjugated with MORF, a phosphorodiamidate morpholino oligomer) and then targeted by 188Re-cMORF (a 188Re labeled complementary MORF). Before the dose-escalating therapeutic study, a pretargeting study in LS174T tumored mice was performed at tracer levels. By both necropsy and imaging, the tracer study showed that the whole body radioactivity was largely restricted to tumor in the mice pretargeted 48 h earlier with MORF-CC49 and the tumor radioactivity was retained over 90 h. After decay correction, a best-fit to the biodistribution provided the areas under the radioactivity curves (AUCs) used for the radiation dose estimates. The tumor to normal organ AUC ratios in all cases were greater than unity and ranged from 3 (kidneys) to 48 (muscle). Tumor growth was inhibited in the therapy study. At the highest 188Re dose of 1.40 mCi, a complete but temporary tumor remission was evident in 3 out of the 5 animals. Histological examination of tissues from these animals showed no evidence of cytotoxicity to normal tissues but obvious radiation damage to tumor. In conclusion, effective radiotherapy was achieved in a mouse model by MORF/cMORF pretargeting using 188Re as the therapeutic radionuclide and CC49 as the pretargeting antibody.

Melanin-covered nanoparticles for protection of bone marrow during radiation therapy of cancer

PURPOSE

Protection of bone marrow against radiotoxicity during radioimmunotherapy and in some cases external beam radiation therapy such as hemi-body irradiation would permit administration of significantly higher doses to tumors, resulting in increased efficacy and safety of treatment. Melanin, a naturally occurring pigment, possesses radioprotective properties. We hypothesized that melanin, which is insoluble, could be delivered to the bone marrow by intravenously administrated melanin-covered nanoparticles (MNs) because of the human body's "self-sieving" ability, protecting it against ionizing radiation.

METHODS AND MATERIALS

The synthesis of MNs was performed via enzymatic polymerization of 3,4-dihydroxyphenylalanine and/or 5-S-cysteinyl-3,4-dihydroxyphenylalanine on the surface of 20-nm plain silica nanoparticles. The biodistribution of radiolabeled MNs in mice was done at 3 and 24 h. Healthy CD-1 mice (Charles River Laboratories International, Inc., Wilmington, MA) or melanoma tumor-bearing nude mice were given MNs intravenously, 50 mg/kg of body weight, 3 h before either whole-body exposure to 125 cGy or treatment with 1 mCi of (188)Re-labeled 6D2 melanin-binding antibody.

RESULTS

Polymerization of melanin precursors on the surface of silica nanoparticles resulted in formation of a 15-nm-thick melanin layer as confirmed by light scattering, transmission electron microscopy, and immunofluorescence. The biodistribution after intravenous administration showed than MN uptake in bone marrow was 0.3% and 0.2% of injected dose per gram at 3 and 24 h, respectively, whereas pre-injection with pluronic acid increased the uptake to 6% and 3% of injected dose per gram, respectively. Systemic MN administration reduced hematologic toxicity in mice treated with external radiation or radioimmunotherapy, whereas no tumor protection by MNs was observed.

CONCLUSIONS

MNs or similar structures provide a novel approach to protection of bone marrow from ionizing radiation based on prevention of free radical formation by melanin.

Algorithms to predict cerebral malaria in murine models using the SHIRPA protocol

Background

Plasmodium berghei ANKA infection in C57Bl/6 mice induces cerebral malaria (CM), which reproduces, to a large extent, the pathological features of human CM. However, experimental CM incidence is variable (50-100%) and the period of incidence may present a range as wide as 6-12 days post-infection. The poor predictability of which and when infected mice will develop CM can make it difficult to determine the causal relationship of early pathological changes and outcome. With the purpose of contributing to solving these problems, algorithms for CM prediction were built.

Methods

Seventy-eight P. berghei-infected mice were daily evaluated using the primary SHIRPA protocol. Mice were classified as CM+ or CM- according to development of neurological signs on days 6-12 post-infection. Logistic regression was used to build predictive models for CM based on the results of SHIRPA tests and parasitaemia.

Results

The overall CM incidence was 54% occurring on days 6-10. Some algorithms had a very good performance in predicting CM, with the area under the receiver operator characteristic (_auROC) curve ≥ 80% and positive predictive values (PV+) ≥ 95, and correctly predicted time of death due to CM between 24 and 72 hours before development of the neurological syndrome (_auROC = 77-93%; PV+ = 100% using high cut off values). Inclusion of parasitaemia data slightly improved algorithm performance.

Conclusion

These algorithms work with data from a simple, inexpensive, reproducible and fast protocol. Most importantly, they can predict CM development very early, estimate time of death, and might be a valuable tool for research using CM murine models.

Melanin-targeted preclinical PET imaging of melanoma metastasis

Dialkylamino-alkyl-benzamides possess an affinity for melanin, suggesting that labeling of such benzamides with (18)F could potentially produce melanin-targeted PET probes able to identify melanotic melanoma metastases in vivo with high sensitivity and specificity.

METHODS

In this study, N-[2-(diethylamino)ethyl]-4-(18)F-fluorobenzamide ((18)F-FBZA) was synthesized via a 1-step conjugation reaction. The sigma-receptor binding affinity of (19)F-FBZA was determined along with the in vitro cellular uptake of radiofluorinated (18)F-FBZA in B16F10 cells. In vivo distribution and small-animal PET studies were conducted on mice bearing B16F10 melanoma, A375M amelanotic melanoma, and U87MG tumors, and comparative studies were performed with (18)F-FDG PET in the melanoma models.

RESULTS

In vitro, uptake of (18)F-FBZA was significantly higher in B16F10 cells treated with l-tyrosine (P < 0.001). In vivo, (18)F-FBZA displayed significant tumor uptake; at 2 h, 5.94 +/- 1.83 percentage injected dose (%ID) per gram was observed in B16F10 tumors and only 0.75 +/- 0.09 %ID/g and 0.56 +/- 0.13 %ID/g was observed in amelanotic A375M and U87MG tumors, respectively. Lung uptake was significantly higher in murine lungs bearing melanotic B16F10 pulmonary metastases than in normal murine lungs (P < 0.01). Small-animal PET clearly identified melanotic lesions in both primary and pulmonary metastasis B16F10 tumor models. Coregistered micro-CT with small-animal PET along with biopsies further confirmed the presence of tumor lesions in the mouse lungs.

CONCLUSION

(18)F-FBZA specifically targets primary and metastatic melanotic melanoma lesions with high tumor uptake and may have translational potential.

Radioimmunotherapy of experimental human metastatic melanoma with melanin-binding antibodies and in combination with dacarbazine

PURPOSE

Melanin has emerged as an attractive target for radioimmunotherapy (RIT) of melanoma, and a radiolabeled monoclonal antibody (mAb) 6D2 to melanin is currently in clinical evaluation. We investigated two approaches to improve the targeting of radiation to tumors using melanin-binding mAbs: (a) the use of an additional mAb to melanin could provide information on whether using antibodies to melanin can serve as a general approach to development of therapeutics for melanoma, and (b) as melanin targeting involves the antibody binding to extracellular melanin released from necrotic melanoma cells, we hypothesized that the administration of a chemotherapeutic agent followed by RIT would facilitate the delivery of radiation to the tumors due to the increased presence of free melanin.

EXPERIMENTAL DESIGN

We evaluated the therapeutic efficacy of two melanin-binding IgM mAbs labeled with (188)Re (6D2 and 11B11). We compared the efficacy of RIT with (188)Re-6D2 to chemotherapy with dacarbazine (DTIC) and to combined chemotherapy and RIT in human metastatic melanoma-bearing nude mice.

RESULTS

Therapeutic efficacy of (188)Re-labeled 6D2 and 11B11 was comparable despite differences in their affinity and binding site numbers. Comparison of chemotherapy with DTIC and RIT revealed that RIT was more effective in slowing tumor growth in mice. Administration of DTIC followed by RIT was more effective than either modality alone.

CONCLUSIONS

These results provide encouragement for the development of RIT for melanoma with melanin-binding mAbs and suggest that combining chemotherapy and RIT may be a promising approach for the treatment of metastatic melanoma.

Peptide-targeted radionuclide therapy for melanoma

Melanocortin-1 receptor (MC1-R) and melanin are two attractive melanoma-specific targets for peptide-targeted radionuclide therapy for melanoma. Radiolabeled peptides targeting MC1-R/melanin can selectively and specifically target cytotoxic radiation generated from therapeutic radionuclides to melanoma cells for cell killing, while sparing the normal tissues and organs. This review highlights the recent advances of peptide-targeted radionuclide therapy of melanoma targeting MC1-R and melanin. The promising therapeutic efficacies of 188Re-(Arg(11))CCMSH (188Re-[Cys(3,4,10), D-Phe(7),Arg(11)]-alpha-MSH(3-13)), 177Lu- and 212Pb-labeled DOTA-Re(Arg(11))CCMSH (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-[ReO-(Cys(3,4,10), D-Phe(7), Arg(11))]-alpha-MSH(3-13)) and 188Re-HYNIC-4B4 (188Re-hydrazinonicotinamide-Tyr-Glu-Arg-Lys-Phe-Trp-His-Gly-Arg-His) in preclinical melanoma-bearing models demonstrate an optimistic outlook for peptide-targeted radionuclide therapy for melanoma. Peptide-targeted radionuclide therapy for melanoma will likely contribute in an adjuvant setting, once the primary tumor has been surgically removed, to treat metastatic deposits and for treatment of end-stage disease. The lack of effective treatments for metastatic melanoma and end-stage disease underscores the necessity to develop and implement new treatment strategies, such as peptide-targeted radionuclide therapy.

Pharmacokinetics and dosimetry of 188 Re-pharmaceuticals

The main objective of this review is to apportion current and new insight into the biodistribution, radiopharmacokinetics, dosimetry and cell targeting of rhenium-188 labeled radiopharmaceuticals used as therapeutic drugs. The emphasis lies on the generator obtained rhenium-188, its physical, therapeutic, dosimetric and coordinated compounds. Its use in radioimmunotherapy for lymphoma and other hematological diseases with monoclonal antibodies is discussed. Radiolabeled peptides to target cell receptors are an important field in nuclear medicine and in some research facilities are already being used, especially, somatostatin, bombesin and other peptides. Small molecules labeled with 188 Re are promising as therapeutic drugs. A review about some of the non-specific targeting molecules with therapeutic or pain palliation effect such as phosphonates, lipiodol, microparticles and other interesting molecules is included. Research on the labeling of biomolecules with the versatile rhenium-188 has contributed to the development of therapeutics with favorable pharmacokinetic and dosimetric properties for cancer treatment.

Computational model predicts effective delivery of 188-Re-labeled melanin-binding antibody to metastatic melanoma tumors with wide range of melanin concentrations

Metastatic melanoma is almost always deadly and new methods of treatment are urgently needed. Recently, we established the feasibility of radioimmunotherapy (RIT) for experimental melanoma in mice using a 188-rhenium (188Re)-labeled monoclonal antibody (mAb) 6D2 (IgM) to melanin. Our objective was to determine the effects of varying tumor melanin concentration and of different diffusivities and lymphatic clearance rates of the normal tissue, on the absorbed dose to the tumor in simulated therapy, in preparation for a clinical trial of RIT for melanoma. Using finite element analysis (FEA), we created a pharmacokinetic model that describes melanin-targeting RIT of a melanoma micrometastasis (1.3-mm radius) imbedded in normal tissue (14.3-mm radius). Our method incorporates antibody plasma kinetics, transcapillary transport, interstitial diffusion, and lymphatic clearance. Michaelis-Menten kinetics was used to model mAb binding to tumor melanin for melanin concentrations of 76, 7.6, 0.76, 0.076, and 0.0076 micromol/l. An absorbed dose was calculated, after accounting for direct and crossfire irradiation, on the basis of a 7.4-GBq intravenous dose of 188Re-6D2. The results showed that penetration of mAb into the tumor was inversely proportional to tumor melanin concentration. Decreased diffusivity and increased lymphatic clearance of the surrounding normal tissue decreased the dose to the tumor. The formation of mAb-melanin complex was remarkably similar within a 1000-fold range of melanin concentration, resulting in total doses of 2840, 2820, 2710, and 1990 cGy being delivered to tumors with melanin concentrations of 76, 7.6, 0.76, and 0.076 micromol/l, respectively. In conclusion, RIT of metastatic melanoma can be effective over a wide range of tumor melanin concentrations. The results can be useful in the design of a clinical trial of melanin-targeting RIT in patients with metastatic melanoma.

Phage display library derived peptides that bind to human tumor melanin as potential vehicles for targeted radionuclide therapy of metastatic melanoma

Metastatic melanoma remains an incurable disease, and there is a great need for novel therapeutic modalities. We have recently identified melanin as a target for radionuclide therapy of melanoma and demonstrated the feasibility of this approach using a 188-rhenium ( (188)Re)-radiolabeled melanin-binding decapeptide to fungal melanin known as 4B4. Although the results indicated that radiolabeled melanin-binding decapeptide had activity against melanoma, that peptide also manifested high kidney uptake and this might become a concern during clinical trials. We hypothesized that by identifying peptides with different amino acid composition against tumor melanin we might be able to decrease their kidney uptake. Using the Heptapeptide Ph.D.-7 Phage Display Library, we identified three heptapeptides that bind to human tumor melanin. These peptides were radiolabeled with (188)Re via HYNIC ligand, and their comprehensive biodistribution in A2058 human metastatic melanoma tumor-bearing nude mice was compared to that of (188)Re-4B4 decapeptide. While tumor uptake of heptapeptides was quite similar to that of (188)Re-4B4 decapeptide, there was dramatically less uptake in the kidneys at both 3 h (6% ID/g vs 38%) and 24 h (2% ID/g vs 15%) postinjection. Administration of one of the generated heptapeptides, (188)Re-HYNIC-AsnProAsnTrpGlyProArg, to A2058 human metastatic melanoma-bearing nude mice resulted in significant retardation of the tumor growth. Immunofluorescence showed that in spite of their relatively small size heptapeptides were not able to penetrate through the membranes of viable melanoma cells and bound only to extracellular melanin, which provides assurance that they will be safe to healthy melanin-containing tissues during radionuclide therapy. Thus, these heptapeptides appear to have potentially significant advantages for targeted therapy of melanoma relative to existing melanin-binding peptides.

MO tripeptide diastereomers (M=99/99mTc, Re): models to identify the structure of 99mTc peptide targeted radiopharmaceuticals

Biologically active molecules, such as many peptides, serve as targeting vectors for radiopharmaceuticals based on 99mTc. Tripeptides can be suitable chelates and are easily and conveniently synthesized and linked to peptide targeting vectors through solid-phase peptide synthesis and form stable TcVO complexes. Upon complexation with [TcO]3+, two products form; these are syn and anti diastereomers, and they often have different biological behavior. This is the case with the approved radiopharmaceutical [99mTcO]depreotide ([99mTcO]P829, NeoTect) that is used to image lung cancer. [99mTcO]depreotide indeed exhibits two product peaks in its HPLC profile, but assignment of the product peaks to the diastereomers has proven to be difficult because the metal peptide complex is difficult to crystallize for structural analysis. In this study, we isolated diastereomers of [99TcO] and [ReO] complexes of several tripeptide ligands that model the metal chelator region of [99mTcO]depreotide. Using X-ray crystallography, we observed that the early eluting peak (A) corresponds to the anti diastereomer, where the Tc=O group is on the opposite side of the plane formed by the ligand backbone relative to the pendant groups of the tripeptide ligand, and the later eluting peak (B) corresponds to the syn diastereomer, where the Tc=O group is on the same side of the plane as the residues of the tripeptide. 1H NMR and circular dichroism (CD) spectroscopy report on the metal environment and prove to be diagnostic for syn or anti diastereomers, and we identified characteristic features from these techniques that can be used to assign the diastereomer profile in 99mTc peptide radiopharmaceuticals like [99mTcO]depreotide and in 188Re peptide radiotherapeutic agents. Crystallography, potentiometric titration, and NMR results presented insights into the chemistry occurring under physiological conditions. The tripeptide complexes where lysine is the second amino acid crystallized in a deprotonated metallo-amide form, possessing a short N1-M bond. The pKa measurements of the N1 amine (pKa approximately 5.6) suggested that this amine is rendered more acidic by both metal complexation and the presence of the lysine residue. Furthermore, peptide chelators incorporating a lysine (like the chelator of [TcO]depreotide) likely exist in the deprotonated form in vivo, comprising a neutral metal center. Deprotonation possibly mediates the interconversion process between the syn and anti diastereomers. The N1 amine group on non-lysine-containing metallopeptides is not as acidic (pKa approximately 6.8) and does not deprotonate and crystallize as do the metallo-amide species. Three of the tripeptide ligands (FGC, FSC, and FKC) were radiolabeled with 99mTc, and the individual syn and anti isomers were isolated for biodistribution studies in normal female nude mice. The main organs of uptake were the liver, intestines, and kidneys, with the FGC compounds exhibiting the highest liver uptake. In comparing the diastereomers, the syn compounds had substantially higher organ uptake and slower blood clearance than the anti compounds.

Targeting the Virus with Radioimmunotherapy in Virus-Associated Cancers

"Virus-associated cancer" (VAC) refers to a cancer where viral infection results in the malignant transformation of the host's infected cells. Examples of viruses linked to cancers are the Epstein-Barr virus (EBV), which is associated with lymphomas, as well as nasopharyngeal and breast cancer; hepatitis B virus (HBV) and hepatitis C virus (HCV), which are both associated with hepatocellular carcinoma; and human papilloma viruses (HPVs), which are associated with cancer of the cervix. We have recently demonstrated that HIV-1-infected cells can be eliminated in vitro and in vivo by targeting viral glycoproteins expressed on the surface of infected cells with radiolabeled viral protein-specific monoclonal antibodies and proposed that this approach can be applicable to the broad range of viral infectious diseases. In VAC, the tumor cells can exhibit viral antigens both internally or on their surfaces. As a result, viral antigens in tumors represent a potential antigenic target that is clearly different from normal tissues. In principle, these proteins could be targeted by radioimmunotherapy (RIT). In this paper, we describe the potential of this approach and review some of the issues involved in the development of this approach. RIT of VAC is fundamentally different from the previously described uses of RIT, which have targeted tumor-associated antigens that are "self" proteins.

Update: Renaissance of Targeting Molecules for Melanoma

Malignant melanoma affects approximately 40,000 new patients each year in the United States and an estimated 100,000 people worldwide. There is no satisfactory treatment for patients with metastatic melanoma that have an estimated 5-year survival of 6%. The potential of radioimmunotherapy (RIT) for the treatment of metastatic melanoma was recognized very early by RIT pioneers when murine melanoma was successfully treated by DeNardo, and later when Larson reported a shrinkage of tumor in a patient with metastatic melanoma treated with 131I-labeled Fab' fragments of a mAb against high-molecular-weight melanoma-associated antigen. Despite successes in the 1980s, RIT of melanoma did not develop into a clinical modality. The reasons for this are complex. In recent years, RIT has made an impression, as evidenced by the recent approval of Zevalin and Bexxar (anti-CD20 mAbs labeled with 90Y and 131I, respectively). Now there is a "window of opportunity" for RIT to become an effective therapy for metastatic melanoma. Surface antigen GD3 has been evaluated in patients as a potential target for melanoma RIT; pretargeting the administration of antibodies and intralesional administration of an antibody labeled with potent alpha-emitter 213-Bismuth have shown promise in clinical studies. Melanin, the pigment that gives melanoma its name, has emerged as a novel antigen for delivery of radioactivity to the tumors by antimelanin antibody. Simultaneously, radiolabeled metal-cyclized alpha-MSH peptide analogs and melanin-binding peptides are being developed as targeting molecules for melanoma. Overall, we are hopeful that targeted radionuclide therapy of metastatic melanoma will become a reality within the next few years.