Synthesis and nca-radioiodination of arylstannyl-cobalamin conjugates. Evaluation of aryliodo-cobalamin conjugate binding to transcobalamin II and biodistribution in mice

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Evolution of Cancer Pharmacological Treatments at the Turn of the Third Millennium

The medical history of cancer began millennia ago. Historical findings of patients with cancer date back to ancient Egyptian and Greek civilizations, where this disease was predominantly treated with radical surgery and cautery that were often ineffective, leading to the death of patients. Over the centuries, important discoveries allowed to identify the biological and pathological features of tumors, without however contributing to the development of effective therapeutic approaches until the end of the 1800s, when the discovery of X-rays and their use for the treatment of tumors provided the first modern therapeutic approach in medical oncology. However, a real breakthrough took place after the Second World War, with the discovery of cytotoxic antitumor drugs and the birth of chemotherapy for the treatment of various hematological and solid tumors. Starting from this epochal turning point, there has been an exponential growth of studies concerning the use of new drugs for cancer treatment. The second fundamental breakthrough in the field of oncology and pharmacology took place at the beginning of the '80s, thanks to molecular and cellular biology studies that allowed the development of specific drugs for some molecular targets involved in neoplastic processes, giving rise to targeted therapy. Both chemotherapy and target therapy have significantly improved the survival and quality of life of cancer patients inducing sometimes complete tumor remission. Subsequently, at the turn of the third millennium, thanks to genetic engineering studies, there was a further advancement of clinical oncology and pharmacology with the introduction of monoclonal antibodies and immune checkpoint inhibitors for the treatment of advanced or metastatic tumors, for which no effective treatment was available before. Today, cancer research is always aimed at the study and development of new therapeutic approaches for cancer treatment. Currently, several researchers are focused on the development of cell therapies, anti-tumor vaccines, and new biotechnological drugs that have already shown promising results in preclinical studies, therefore, in the near future, we will certainly assist to a new revolution in the field of medical oncology.

Single photon emission computed tomography tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g. peptide, antibody fragment) and a γ-radiation emitting radionuclide (e.g. (99m)Tc, (123)I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Structural study on ligand specificity of human vitamin B12 transporters

Studies comparing the binding of genuine cobalamin (vitamin B12) to that of its natural or synthetic analogues have long established increasing ligand specificity in the order haptocorrin, transcobalamin and intrinsic factor, the high-affinity binding proteins involved in cobalamin transport in mammals. In the present study, ligand specificity was investigated from a structural point of view, for which comparative models of intrinsic factor and haptocorrin are produced based on the crystal structure of the homologous transcobalamin and validated by results of published binding assays. Many interactions between cobalamin and its binding site in the interface of the two domains are conserved among the transporters. A structural comparison suggests that the determinant of specificity regarding cobalamin ligands with modified nucleotide moiety resides in the beta-hairpin motif beta3-turn-beta4 of the smaller C-terminal domain. In haptocorrin, it provides hydrophobic contacts to the benzimidazole moiety through the apolar regions of Arg357, Trp359 and Tyr362. Together, these large side chains may compensate for the missing nucleotide upon cobinamide binding. Intrinsic factor possesses only the tryptophan residue and transcobalamin only the tyrosine residue, consistent with their low affinity for cobinamide. Relative affinity constants for other analogues are rationalized similarly by analysis of steric and electrostatic interactions with the three transporters. The structures also indicate that the C-terminal domain is the first site of cobalamin-binding since part of the beta-hairpin motif is trapped between the nucleotide moiety and the N-terminal domain in the final holo-proteins.

Cyanocobalamin (vitamin B12) conjugates with enhanced solubility

Cyanocobalamin (vitamin B12) is an essential nutrient as well as a very useful carrier in drug delivery. Conjugates of vitamin B12 are investigated due to their wide range of therapeutic applications. We report the synthesis of six vitamin B12 conjugates, and the effect of conjugation on their solubilities and stabilities in various media. We reveal here that vitamin B12 can be released readily if a 2'-hydroxyl group is conjugated rather than the 5'-hydroxyl group, and the solubility (thus the equivalents of vitamin B12) could be enhanced as much as 19-fold, by simple conjugates such as glycolates. Findings disclosed here provide insights into the reactivities of vitamin B12 conjugates, the design of future prodrugs and similar conjugated moieties using vitamin B12.

Vitamin-mediated targeting as a potential mechanism to increase drug uptake by tumours

Targeted chemotherapy for cancer treatment offers a great potential advantage in tumour treatment due to greater specificity of delivery which leads to increased dose of the cytotoxin delivered to the tumour relative to the rest of the body. In order to achieve such selective targeted delivery one needs to identify generic markers that are over-expressed on the surface of tumour cells but are not over-expressed on normal tissue. Work of several authors has shown that some cells, such as those of rapidly dividing, aggressive tumours, over-express surface receptors involved in the uptake of vitamin B(12) [B. Rachmilewitz, M. Rachmilewitz, B. Moshkowitz, J. Gross, J. Lab. Clin. Med. 78 (1971) 275-279; B. Rachmilewitz, A. Sulkes, M. Rachmilewitz, A. Fuks, Israel J. Med. Sci. 17 (1981) 874-879] or folate [P. Garin-Chesa, I. Campbell, P.E. Saigo, J.L. Lewis Jr., L.J. Old, W.J. Rettig, Am. J. Pathol. 142 (1993) 557-567; O.C. Boerman, C.C. van Niekerk, K. Makkink, T.G.J.M. Hanselaar, P. Kenemans, L.G. Poels, Int. J. Gynecol. Pathol. 10 (1991) 15-25; G. Toffoli, C. Cernigoi, A. Russo, A. Gallo, M. Bagnoli, M. Boiocchi, Int. J. Cancer 74 (1997) 193-194; J.A. Reddy, D. Dean, M.D. Kennedy, P.S. Low, J. Pharm. Sci. 88 (1999) 1112-1118; J.A. Reddy, P.S. Low, Crit. Rev. Ther. Drug Carrier Syst. 15 (1998) 587-627; G.J. Russell-Jones, K. McTavish, J.F. McEwan, in: Proceedings of the 2nd International Symposium on Tumor Targeted Delivery Systems, 2002]. Furthermore the degree of over-expression has been found to correlate with the stage of tumour growth, with the highest levels found on stage IV carcinomas. Using fluorescently-labelled polymers to which are linked the targeting agents, vitamin B(12), folate or biotin, the relative uptake of these polymers into various types of tumour cell lines grown both in vitro and in vivo has been examined. These studies have shown that while some tumour types do NOT over-express receptors involved in vitamin uptake, most tumour types over-express receptors for folate, or vitamin B(12). In either case there is also a greatly increased expression of a yet to be identified biotin receptor. In cases of receptor over-expression, binding of the targeted fluorochrome leads to rapid internalization of these molecules within the cells to levels that are two to thirty times higher than with non-targeted polymers. Using a number of cancer models, these studies were extended further and it was found that the increased expression of receptors also leads to increased levels of killing with targeted cytotoxins. Thus the preliminary data described suggests that the use of vitamins as targeting agents has enormous potential for use in cancer diagnosis and chemotherapy.

Equilibrium and kinetic analyses of the interactions between vitamin B(12) binding proteins and cobalamins by surface plasmon resonance

Surface plasmon resonance biosensor analysis was used to evaluate the thermodynamics and binding kinetics of naturally occurring and synthetic cobalamins interacting with vitamin B(12) binding proteins. Cyanocobalamin-b-(5-aminopentylamide) was immobilized on a biosensor chip surface to determine the affinity of different cobalamins for transcobalamin, intrinsic factor, and nonintrinsic factor. A solution competition binding assay, in which a surface immobilized cobalamin analog competes with analyte cobalamin for B(12) protein binding, shows that only recombinant human transcobalamin is sensitive to modification of the corrin ring b-propionamide of cyanocobalamin. A direct binding assay, where recombinant human transcobalamin is conjugated to a biosensor chip, allows kinetic analysis of cobalamin binding. Response data for cyanocobalamin binding to the transcobalamin protein surface were globally fitted to a bimolecular interaction model that includes a term for mass transport. This model yields association and dissociation rate constants of k(a) = 3 x 10(7) M(-1) s(-1) and k(d) = 6 x 10(-4) s(-1), respectively, with an overall dissociation constant of K(D) = 20 pM at 30 degrees C. Transcobalamin binds cyanocobalamin-b-(5-aminopentylamide) with association and dissociation rates that are twofold slower and threefold faster, respectively, than transcobalamin binding to cyanocobalamin. The affinities determined for protein-ligand interaction, using the solution competition and direct binding assays, are comparable, demonstrating that surface plasmon resonance provides a versatile way to study the molecular recognition properties of vitamin B(12) binding proteins.

Radioiodination of cyanocobalamin conjugates containing hydrophilic linkers: preparation of a radioiodinated cyanocobalamin monomer and two dimers, and assessment of their binding with transcobalamin II

This report describes an investigation aimed at preparation of radioiodinated cyanocobalamin (CN-Cbl) monomers and dimers with improved water solubility and decreased nonspecific binding. In the investigation, synthesis and radioiodination reactions of one monomeric and two dimeric CN-Cbl derivatives were conducted. The initial step in the synthesis of the CN-Cbl derivatives was mild acid hydrolysis of CN-Cbl, 1, followed by separation of the resultant corrin ring b-, d-, and e-monocarboxylate isomers. The investigation was limited to preparation of conjugates of CN-Cbl-e-carboxylate, 2, as earlier studies had shown binding of that isomer with recombinant human transcobalamin II (rhTCII) was similar to CN-Cbl. In a second synthetic step, the hydrophilic linker moiety, 4,7,10-trioxa-1,13-tridecandiamine, 3, was conjugated with 2 to form the adduct, 4. The synthesis of a monomeric CN-Cbl derivative, 6a, which can be used for radioiodination, was accomplished by reaction of 4 with p-tri-n-butylstannylbenzoate tetrafluorophenyl (TFP) ester, 5a. Two CN-Cbl dimers containing the arylstannane radioiodination moiety were also synthesized. The first dimer, 8a, was synthesized by cross-linking 4 with a stannylbenzoyl-aminoisophthalate di-TFP ester, 7a. The second dimer, 11a, was synthesized by reacting benzene tricarboxylate tri-TFP ester, 10, in a stepwise manner with 1 equiv of the adduct of 5a and 3 (forming 9a), followed by 2 equiv of 4. Iodobenzoate HPLC standards, 6b, 8b, and 11b, used in the radioiodination studies, were prepared in a manner similar to that of the stannylbenzoate derivatives. Radioiodinations were performed by reacting 6a, 8a, or 11a with N-chlorosuccinimide and Na[(125)I]I in methanol under neutral conditions. Radiochemical yields of 17-42% were obtained. Evaluation of the binding properties of radiolabeled CN-Cbl conjugates with rhTCII showed that the dimer of CN-Cbl, 11b, bound more avidly than the monomer, 6b, and that the binding affinity of the dimer is essentially equivalent to that of unmodified CN-Cbl. Incubation of radioiodinated monomer, [(125)I]6b, and dimer, [(125)I]11b, with rhTCII followed by size-exclusion chromatographic analysis provided data that the monomer bound one rhTCII molecule whereas two rhTCII molecules were bound to approximately 30% of the dimer.