Targeted delivery of cytotoxic proteins to prostate cancer via conjugation to small molecule urea-based PSMA inhibitors

Prostate cancer cells are characterized by a remarkably low proliferative rate and the production of high levels of prostate-specific proteases. Protein-based toxins are attractive candidates for prostate cancer therapy because they kill cells via proliferation-independent mechanisms. However, the non-specific cytotoxicity of these potent cytotoxins must be redirected to avoid toxicity to normal tissues. Prostate-Specific Membrane Antigen (PSMA) is membrane-bound carboxypeptidase that is highly expressed by prostate cancer cells. Potent dipeptide PSMA inhibitors have been developed that can selectively deliver and concentrate imaging agents within prostate cancer cells based on continuous PSMA internalization and endosomal cycling. On this basis, we conjugated a PSMA inhibitor to the apoptosis-inducing human protease Granzyme B and the potent Pseudomonas exotoxin protein toxin fragment, PE35. We assessed selective PSMA binding and entrance into tumor cell to induce cell death. We demonstrated these agents selectively bound to PSMA and became internalized. PSMA-targeted PE35 toxin was selectively toxic to PSMA producing cells in vitro. Intratumoral and intravenous administration of this toxin produced marked tumor killing of PSMA-producing xenografts with minimal host toxicity. These studies demonstrate that urea-based PSMA inhibitors represent a simpler, less expensive alternative to antibodies as a means to deliver cytotoxic proteins to prostate cancer cells.

Prostate-specific membrane antigen as a target for cancer imaging and therapy

The prostate-specific membrane antigen (PSMA) is a molecular target whose use has resulted in some of the most productive work toward imaging and treating prostate cancer over the past two decades. A wide variety of imaging agents extending from intact antibodies to low-molecular-weight compounds permeate the literature. In parallel there is a rapidly expanding pool of antibody-drug conjugates, radiopharmaceutical therapeutics, small-molecule drug conjugates, theranostics and nanomedicines targeting PSMA. Such productivity is motivated by the abundant expression of PSMA on the surface of prostate cancer cells and within the neovasculature of other solid tumors, with limited expression in most normal tissues. Animating the field is a variety of small-molecule scaffolds upon which the radionuclides, drugs, MR-detectable species and nanoparticles can be placed with relative ease. Among those, the urea-based agents have been most extensively leveraged, with expanding clinical use for detection and more recently for radiopharmaceutical therapy of prostate cancer, with surprisingly little toxicity. PSMA imaging of other cancers is also appearing in the clinical literature, and may overtake FDG for certain indications. Targeting PSMA may provide a viable alternative or first-line approach to managing prostate and other cancers.

GCPII imaging and cancer

Glutamate carboxypeptidase II (GCPII) in the central nervous system is referred to as the prostate-specific membrane antigen (PSMA) in the periphery. PSMA serves as a target for imaging and treatment of prostate cancer and because of its expression in solid tumor neovasculature has the potential to be used in this regard for other malignancies as well. An overview of GCPII/PSMA in cancer, as well as a discussion of imaging and therapy of prostate cancer using a wide variety of PSMA-targeting agents is provided.

Preparation of F-18 labeled annexin V: a potential PET radiopharmaceutical for imaging cell death

The clinical response to antitumor therapy is measured using imaging, such as CT or MRI, 6-12 weeks following chemotherapy treatment. The images at that time reflect both tumor cell death and new growth. Therefore, the amount of tumor cell death caused by chemotherapy cannot be efficiently quantified with current imaging modalities. A quantitative measurement of tumor cell death immediately following chemotherapy is needed to help validate both new agents and to optimize administration of existing therapies. Annexin V is a 36kD protein that binds to exposed phosphatidylserine (PS) on dying cells. In order to synthesize a probe that can detect cell death in vivo, the positron emitter F-18 was conjugated to annexin V via the compound N- succinimidyl-4-[18F]fluorobenzoate, [18F]SFB. The decay corrected radiochemical yield of F-18 labeled annexin V from 18F fluoride was 17.6 +/- 5.6% (n = 4) in three hours. The stepwise radiochemical yield of the conjugation step with annexin V was as high as 70% when a protein concentration of 5 mg/ml was used. Cancer cells treated with the chemotherapeutic agent, etoposide, showed an 88% increase in the binding of F-18 labeled annexin V compared to untreated cells. We conclude that [18F] labeled annexin V can be readily prepared by the conjugation of annexin V with [18F]SFB and that the positron-emitting compound is biologically active in detecting apoptosis.

Newer methods of labeling diagnostic agents with Tc-99m

The past several years have seen marked advances in technetium/rhenium chemistry applicable to the preparation of new 99mTc-labeled radiopharmaceuticals. This article focuses on recent developments in technetium chemistry, including the preparation of "3 + 1" complexes, the preparation and use of (99mTc[CO]3)+ complexes for labeling biomolecules, the preparation of rhenium steroid inclusion complexes, improvements in both hydrazinonicotinamide labeling chemistry and in the preformed 99mTc complex method of labeling biomolecules, and new solid-phase separation techniques that may allow the isolation of high specific-activity radiopharmaceuticals in a clinical setting.

Trypanosoma musculi: tracking parasites and circulating lymphoid cells in host mice

Two aspects of host-parasite relationships that seem worthy of more attention are: (a) the distribution of parasites among host organs in the early course of infection, and (b) the dynamics of host lymphocyte tissue localization and recirculation during the course of infection. We have employed the derivatized aminostyrylpyridinium dye, [125I] I 2P-Di-6-ASP, to provide a relatively stable tag on both a parasite, Trypanosoma musculi, and on host mouse splenocytes, enriched B and T lymphocytes, and natural killer cells. The organ distribution of the parasites, splenocytes, and lymphocytes in recipient, host mice was tracked. Radiolabeled T. musculi localized primarily in the liver with lesser numbers in spleen, lungs, and kidneys. Per unit wet weight, the spleen accumulated parasites most efficiently. When T. musculi were inoculated intraperitoneally, most of them remained in the peritoneal space and the numbers that gained access to liver, lungs, and spleen were significantly smaller than in mice inoculated intravenously. The acquisition of parasites by the spleen (and lungs) of mice with an existing T. musculi infection was markedly inhibited. This was true also of syngeneic splenocytes and lymphocytes. In addition, lymphocytes from infected mice were significantly less likely to take residence in the spleens of normal recipient mice and were especially unlikely to localize in the spleens of infected recipients. These and other findings suggested that the inability of circulating lymphocytes to gain access to lymphoid tissues in infected mice, coupled with the poor ability of those tissues to sequester parasite antigens, could account for the known prolonged delay in the development of curative antibody response characteristic of T. musculi-infected mice. It is likely that the marked disruption of lymphoid tissue histoarchitecture that is typical of T. musculi infection contributes significantly to the failure of the tissues to sequester parasites and lymphocytes. Because lymphoid tissue disruption is seen in many parasitic infections, the findings reported here may have fairly broad relevance. In any case, the procedure described here for labeling parasites and lymphocytes should be of general utility for tracking their disposition in vivo.

Effects of aging on the dynamics of lymphocyte organ distribution in mice: use of a radioiodinated cell membrane probe

We have employed a derivatized aminostyrylpyridinium dye, [125I]I2P-Di-6-ASP, to provide a relatively stable tag on mixed mouse splenocytes and purified B and T cells for the purpose of tracking the distribution of those cells among the organs of normal young (4 months) and aged (> 26 months) recipient mice. Cells from both young and aged donor spleens were studied. Special emphasis was placed on localization of donor cells in the spleens of the recipients because the majority of circulating lymphocytes localize in the spleen and the spleen is the principal organ of primary immune response. There was a profound difference in the efficiency of splenic acquisition of donor cells between young and aged recipients, a difference not found in the liver, lungs, kidneys or heart. In contrast young and old donor lymphocytes lodged equally well in the spleens of recipients of the same age. It was clear that the competence of the splenic microenvironment to serve as a lodging site for circulating lymphocytes deteriorated with age. Such a change could contribute significantly to the deficient immune response of aged subjects. We suggest that aging results in significant change in the splenic extracellular matrix to serve as an adhesive substratum for lymphocytes. Our data point to a need for detailed studies on age-related changes in components of the extracellular matrix within lymphoid tissues. The novel compound which we employed for cell labeling is both radioactive and fluorescent and should be quite suitable for such studies.

Pre-clinical and clinical studies of two new bifunctional chelating agents for immunoscintigraphy with 111In-anti-CEA monoclonal antibody

Anti-CEA F(ab')2 monoclonal antibody fragments [F6 MAb F(ab')2] were conjugated to two bifunctional semi-rigid chelating agents derived from trans-1,2-diaminocyclohexane tetraacetic acid (CDTA), the monolithium salt of N-[methyl(2-isothiocyanatoethyl)carbamide] trans-1,2-diaminocyclohexane-N,N',N'-triacetic acid (SCN), and 4 isothiocyanato-trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (4-ICE) and labelled with 111In to obtain IIIIn-labelled-F6 MAb F(ab')2 conjugates (111In-F6-SCN and 111In-F6-4-ICE respectively). Biodistribution in mice and clinical studies were undertaken to assess the potential of these two ligands in the detection of colorectal adenocarcinoma recurrences and metastases in humans. Toxicity studies were carried out on guinea pigs and Swiss mice injected with a dose proportionally 100 times greater than that used in human studies. Clinical studies were performed in patients with clinically and/or biologically suspected adenocarcinoma recurrences. No immunoconjugate-induced toxicity was found. The biodistribution studies in mice gave better visualization of tumour sites with 111In-F6-SCN and 111In-F6-4-ICE than with 111In-F6-DTPA. Ten patients were included in the clinical protocol. 111In-F6-SCN and 111In-F6-4-ICE effectively visualized adenocarcinoma recurrences. However, in this small series, 111In-F6-4-ICE performed somewhat better than 111In-F6-SCN. The present study has demonstrated the potential of new bifunctional semi-rigid chelating agents coupled to antibody and labelled with 111In to localize recurrences (especially in liver) in humans using a one-step targeting method.

Radioiodinated (aminostyryl)pyridinium (ASP) dyes: new cell membrane probes for labeling mixed leukocytes and lymphocytes for diagnostic imaging

We prepared [125I/131I]iodo-(aminostyryl)pyridinium dyes from tributylstannyl precursors. ASP 7a and 7b labeled leukocytes ex vivo (70-94%) using saline with or without washing plasma from cells. Viability of peripheral blood lymphocytes (PBLs) (dogs, rats) and splenic lymphocytes (rats) labeled with 7a and 7b (71-82%) was unchanged after labeling (> or = 88%). Canine 7b-leukocytes showed higher uptake in inflammatory lesions than did 111In-oxine leukocytes. At 3 h, aspirates contained more radioiodine than 111In (1.65:1 to 22:1) and radioiodine was cell bound. ROI measurements (3 h) gave abscess to contralateral knee ratios of 12.3 and 10.6 for 131I-7b vs. 4.8 and 2.3 for 111In-oxine.

Introduction of five potentially metabolizable linking groups between 111In-cyclohexyl EDTA derivatives and F(ab')2 fragments of anti-carcinoembryonic antigen antibody--II. Comparative pharmacokinetics and biodistribution in human colorectal carcinoma-bearing nude mice

The five linker-containing immunoconjugates described in the preceding paper were labeled with 111In and tested for their biodistribution, pharmacokinetics and immunoscintigraphic imaging properties in tumor-xenografted nude mice. The results were compared with DTPADA and CDTAMA for reference. Results showed that, for immunoscintigraphy, the derivatives in decreasing order of effectiveness were: aliphatic (tumor/liver > 4.5 and tumor/kidney > 6.5 at 96 h), thioether (tumor/liver > 3 and tumor/kidney > 1.2 at 24 h), ethylene glycol succinate (tumor/liver > 1.7 and tumor/kidney > 0.5 at 24 h) and disulfide (tumor/liver > 0.5 and tumor/kidney > 0.6 at 96 h). Pharmacokinetic results were complementary with those of the biodistribution studies and provide a basis for the study of in vivo metabolic mechanisms of linker-immunoconjugates. Indium-111-labeled linker-immunoconjugates appear promising for tumor imaging with better contrast than what is obtained with the use of the conventional 111In-DTPA dianhydride chelate.

Introduction of five potentially metabolizable linking groups between 111In-cyclohexyl EDTA derivatives and F(ab')2 fragments of anti-carcinoembryonic antigen antibody--I. A new reproducible synthetic method

The purpose of this study was to synthesize new bifunctional linker-chelating agents for the modification of the in vivo distribution of 111In-labeled antibodies. A general simple synthetic method of preparing cyclohexyl EDTA (CDTA) derivatives containing a linker/spacer group is described. Linkers prepared included a diester, a six carbon aliphatic chain, two thioethers and a disulfide group. The CDTA-linker compounds were coupled to F(Ab')2 fragments of anti-carcinoembryonic antigen monoclonal antibody and labeled with 111In with good retention of immunoreactivity.

Progress in research on ligands, nuclides and techniques for labeling monoclonal antibodies

This paper reviews the current methods for radiolabelling monoclonal antibodies with particular emphasis on radiometals useful for radioimmunoscintigraphy. The discussion, however, is equally applicable to therapeutic radionuclides. The advantages and the pitfalls of the various techniques are critically evaluated. Both direct labeling methods, as well as indirect methods using the bifunctional chelating agent approach, are covered. Recent work on the development and synthesis of new and more specific chelating agents, including the approach of utilizing rigid polyaminocarboxylates, is described. Preliminary promising results with these newer generation chelating agents are presented.

Synthesis, purification and stability of no carrier added radioiodinated 1,1-bis(4-hydroxyphenyl)-2-iodo-2-phenylethylene (IBHPE), a prototype triphenylethylene estrogen-receptor binding radiopharmaceutical

A triphenylethylene compound [1,1-bis(4-hydroxyphenyl)-2-iodo-2-phenylethylene; IBHPE] has been labeled by halodestannylation with 123I at a specific radioactivity of 13,200 Ci/mmol (by in vitro receptor assay) after HPLC purification. The corresponding 80mBr-labeled compound (BrBHPE), which has a 3-fold higher affinity for the estrogen receptor, was previously prepared and examined as a potential therapeutic radiopharmaceutical exploiting Auger electron toxicity. Stability of IBHPE was a concern because free iodide was generated when HPLC solvents were removed with a stream of nitrogen in a glass vial; however, decomposition was minimal when polypropylene vials were used, and ethanol solutions of [123I]IBHPE were stable for several days at 0-4 degrees C. Tissue distribution studies of IBHPE after intraperitoneal injection to mature female rats showed highest estradiol-inhibitable uptake in the peritoneal estrogen-receptor rich tissues (uterus, ovaries and vagina) at 30 min. Specific uptake (percent dose per gram) in the pituitary, and peritoneal target tissue-to-blood ratios were greater at 2 h than 30 min. In immature female rats, uterus-to-blood ratios of greater than 50, progressively lowered by increasing diethylstilbestrol levels, were obtained. These data demonstrate good binding of IBHPE to the estrogen receptor in vivo, in spite of extensive non specific binding in in vitro estrogen receptor assays. Most of the label in the uterus at 1 h after injection was still unchanged IBHPE. Our results suggest that IBHPE or related 123I-labeled iodovinyl triphenylethylenes could have diagnostic or therapeutic radiopharmaceutical utility.

Production of no carrier added 80mBr for investigation of Auger electron toxicity

80mBr (half-life = 4.43 h) is an Auger electron emitting nuclide with convenient properties for investigating Auger electron cytotoxicity and with potential for labeling in vivo radiotherapeutic agents. We have investigated three cyclotron target systems capable of generating 80mBr of sufficiently high specific radioactivity (no carrier added) for biomedical experiments. A 83Kr gas target irradiated with 21.5 MeV deuterons made 80mBr at a production yield of 1.6 +/- 0.2 mCi/muAh at saturation. A five-fold increase in 80mBr yield was obtained from 15 MeV proton irradiation of thin elemental Se enriched in 80Se targets although technical improvements are expected to further raise this production yield. This route is therefore superior for current medical cyclotrons. Irradiation of a reusable 80Se copper selenide target also yielded multi-millicurie amounts of 80mBr, and recovery of radiobromine by dry distillation is faster and more convenient than in the elemental Se target, but an optimum copper selenide target for 80mBr production has not yet been built.

Comparison of the distribution of bromine-77-bromovinyl steroidal and triphenylethylene estrogens in the immature rat

The specific uptake and distribution of steroidal and non-steroidal [77Br]bromovinylestrogens were studied in immature female rats to assess the potential of these radioligands for imaging or therapy of estrogen receptor (ER) positive cancers. E-17 alpha [77Br]bromovinylestradiol, its 3 methyl ether and 11 beta-methoxy derivative, as well as the triphenylethylene estrogen, 1,1-bis[4-hydroxy-phenyl]-2-[77Br]bromo-2- phenylethylene all showed diethylstilbestrol inhibitable, specific uptake of radiobromine between 2 and 16 hr after i.p. administration. The highest concentrations in the estrogen target tissues and the highest target tissue-to-blood ratios were found with E-17 alpha-[Br]bromovinyl-11 beta-methoxyestradiol, but it also had rather high nonspecific uptake in all tissues. The triphenylethylene estrogen showed comparable specific uptake in estrogen target tissues to 17 alpha [77Br]bromovinylestradiol at 2 hr but better apparent retention, indicated by higher specific target tissue levels at the later time points. Thus, [77Br]bromovinyl-11 beta- methoxyestradiol and 1,1-bis[4-hydroxyphenyl]-2-[77Br]bromo-2-phenylethylene appear most favorable for these applications.

Bromine-80m radiotoxicity and the potential for estrogen receptor-directed therapy with auger electrons

While theoretically feasible, estrogen receptor (ER)-directed radiotherapy of hormone-dependent cancers has not been realized because no ER-seeking ligand with an appropriate radiotoxic potential has been identified. Since an appropriate nuclide is a key component we studied the 4.4-h half-life, Auger electron-emitting nuclide bromine-80m. When incorporated into DNA this nuclide was radiotoxic to cells in culture and caused substantial chromosomal damage, while similar concentrations of bromine-80m as bromide or bromoantipyrine were without effect. The mean lethal dose for bromine-80m was 45 atoms per nucleus which is consistent with use in receptor-positive cancers with limited numbers of ER.

Estrogen receptor binding affinity and uterotrophic activity of triphenylhaloethylenes

Radiohalogenated estrogens have considerable potential for estrogen receptor-directed imaging and therapy for cancers which contain such receptors. In an effort to evaluate the potential of the triphenyl ethylene structure for such purposes we have synthesized 3 series of 2-halosubstituted triphenylethylenes containing oxygen functions in the 4 position of both aromatic rings attached to carbon 1 of the ethylene and tested their uterotrophic activity and competition for rat uterine low salt extractable, "cytosol" estrogen receptor. Most active, both as competitors for estradiol binding to estrogen receptors and by their ability to stimulate uterine growth are the 1,1-bis-4-hydroxyphenyl derivatives although the 1,1-bis-4-acetoxyphenyl derivatives also show good receptor affinity and demonstrate uterotrophic activities. However, since uterine cytosol contains enzymes which hydrolyze the acetates to the free phenols even during the incubation in the cold used for the competitive binding studies, a significant portion of the competition shown by the diacetates is probably due to their hydrolysis products, the free phenols. The 1,1-bis-4-methoxyphenyl derivatives are weak competitive binders and demonstrate uterotrophic activity only when administered at the higher, 20 micrograms, doses. Comparing the relative activities of various halogens at the 2 position, in each series the bromo and chloro derivatives generally were of similar activity and significantly more active than the corresponding iodo derivative. The non-halogen substituted derivatives were very good competitors for estrogen receptor binding but less active with regard to uterine growth stimulation, providing evidence that in vivo the vinyl halides would appear to be relatively stable to simple dehalogenation. Since they show reasonably good apparent affinities for the estrogen receptor and apparent in vivo stability, reflected by estrogenic activity, these halogen substituted triphenylethylene derivatives appear to be promising substrates for investigations of estrogen receptor directed imaging and therapy.

Bromine-80m-labeled estrogens: Auger electron-emitting, estrogen receptor-directed ligands with potential for therapy of estrogen receptor-positive cancers

To assess their possible use for estrogen receptor (ER)-directed radiotherapy of estrogen receptor-containing cancers, two estrogens were synthesized with the Auger electron-emitting nuclide bromine-80m and administered to immature female rats. Both the triphenylethylene-based estrogen, [80mBr]-2-bromo-1,1-bis(4-hydroxyphenyl)phenylethylene (Br-BHPE) and the steroidal estrogen [80mBr]17 alpha-bromovinylestradiol, showed substantial diethylstilbestrol-inhibitable localization only in the estrogen target tissues, the uterus, pituitary, ovaries, and vagina and, except for the liver and intestines, generally lower concentrations in all other tissues at both 0.5 and 2 h. The [80mBr]Br-BHPE (specific activity, 8700 Ci/mmol), was shown to bind specifically to the low salt extractable ER of the rat uterus. Comparing i.p., i.v., and s.c. administration of [80mBr]BHPE the i.p. route was found to be particularly advantageous to effect maximum, DES-inhibitable concentrations of radiobromine in the ER-rich target organs in the peritoneal cavity. When the tissue distribution of the [80mBr]Br-BHPE was compared with that of sodium bromide-80m, it was apparent that no substantial amounts of radiobromine were released from the bromoestrogen prior to its target tissue localization. The substantial concentration of these bromine-80m-labeled estrogens in ER-rich tissues, combined with previously reported evidence for the effective radiotoxicity of Auger electron-emitting nuclides within cell nuclei suggest a good potential for such ligands for therapy of ER positive cancers.

The synthesis of non-steroidal estrogen receptor binding compounds labeled with 80mBr

Estrogen receptor (ER) binding radiopharmaceuticals have potential for use in the diagnosis and treatment of cancers of the female reproductive system. Two triphenylethylene derivatives based on the structure of hydroxytamoxifen 4, a high ER binding metabolite of tamoxifen 5, have been prepared: 1-(4-dimethylaminoethoxy)phenyl]-1-(4-hydroxy)phenyl-2-bromo-2-phenyl ethylene 2 and 1,1-bis (p-hydroxyphenyl)-2-bromo-2-phenylethylene 3. Both 2 and 3 bind strongly to the ER. Compound 3 has been labeled in modest yield by direct bromination with 80mBr, which was produced by the 83Kr (d,n alpha) reaction. Radiolabeled 22, a dimethoxy precursor of 3, has been prepared in yields ranging between 40 and 60% by a bromination destannylation reaction.