Phase I trial of 177lutetium-labeled J591, a monoclonal antibody to prostate-specific membrane antigen, in patients with androgen-independent prostate cancer

177Lu-Labeled Phosphoramidate-Based PSMA Inhibitors: The Effect of an Albumin Binder on Biodistribution and Therapeutic Efficacy in Prostate Tumor-Bearing Mice

Prostate-specific membrane antigen (PSMA) continues to be an active biomarker for small-molecule PSMA-targeted imaging and therapeutic agents for prostate cancer and various non-prostatic tumors that are characterized by PSMA expression on their neovasculature. One of the challenges for small-molecule PSMA inhibitors with respect to delivering therapeutic payloads is their rapid renal clearance. In order to overcome this pharmacokinetic challenge, we outfitted a ¹⁷⁷Lu-labeled phosphoramidate-based PSMA inhibitor (CTT1298) with an albumin-binding motif (CTT1403) and compared its in vivo performance with that of an analogous compound lacking the albumin-binding motif (CTT1401). The radiolabeling of CTT1401 and CTT1403 was achieved using click chemistry to connect ¹⁷⁷Lu-DOTA-N₃ to the dibenzocyclooctyne (DBCO)-bearing CTT1298 inhibitor cores. A direct comparison in vitro and in vivo performance was made for CTT1401 and CTT1403; the specificity and efficacy by means of cellular uptake and internalization, biodistribution, and therapeutic efficacy were determined for both compounds. While both compounds displayed excellent uptake and rapid internalization in PSMA+ PC3-PIP cells, the albumin binding moiety in CTT1403 conferred clear advantages to the PSMA-inhibitor scaffold including increased circulating half-life and prostate tumor uptake that continued to increase up to 168 h post-injection. This increased tumor uptake translated into superior therapeutic efficacy of CTT1403 in PSMA+ PC3-PIP human xenograft tumors.

Metal-Based PSMA Radioligands

Prostate cancer is one of the most common malignancies for which great progress has been made in identifying appropriate molecular targets that would enable efficient in vivo targeting for imaging and therapy. The type II integral membrane protein, prostate specific membrane antigen (PSMA) is overexpressed on prostate cancer cells in proportion to the stage and grade of the tumor progression, especially in androgen-independent, advanced and metastatic disease, rendering it a promising diagnostic and/or therapeutic target. From the perspective of nuclear medicine, PSMA-based radioligands may significantly impact the management of patients who suffer from prostate cancer. For that purpose, chelating-based PSMA-specific ligands have been labeled with various diagnostic and/or therapeutic radiometals for single-photon-emission tomography (SPECT), positron-emission-tomography (PET), radionuclide targeted therapy as well as intraoperative applications. This review focuses on the development and further applications of metal-based PSMA radioligands.

Post-therapeutic dosimetry of 177Lu-DKFZ-PSMA-617 in the treatment of patients with metastatic castration-resistant prostate cancer

OBJECTIVE

Lu-DKFZ-PSMA-617, a urea-based compound, binds to the extracellular domain of prostate-specific membrane antigen, thus providing an effective target for the treatment of metastatic castration-resistant prostate cancer (mCRPC). Before its therapeutic use, it is necessary that the radiation dosimetry of this radiopharmaceutical be studied to determine the safe activity that can be administered in patients to prevent haematological, renal and liver toxicity. The present study thus aimed to assess the pharmacokinetics and dosimetry of Lu-DKFZ-PSMA-617 in CRPC patients.

MATERIALS AND METHODS

After obtaining ethical clearance from the institute ethics review board, we enrolled mCRPC patients who were positive on a Glu-NH-CO-NH-Lys-(Ahx)-[Ga(HBED-CC)] PET/CT scan. For kidney protection, a cocktail of lysine and arginine diluted in 2 litres of normal saline was infused, starting from 30 to 60 min before Lu-DKFZ-PSMA-617 infusion. The mean administered activity in the overall population was 2.52±1.3 GBq. For the purpose of dosimetry, each patient underwent nine planar whole-body scans along with blood and urine sample collection at 0.5, 3.5, 24, 48, 72, 96, 120, 144 and 168 h, respectively. SPECT/CT was performed to derive the volume of salivary glands (parotid and submandibular glands) and tumour. Dosimetric evaluation was carried out using the OLINDA/EXM 1.0 software.

RESULTS

A total of 26 mCRPC patients with a mean age of 66.30±9.95 years (range: 38-81 years) were recruited. Normal physiological uptake was observed in all the patients in the lacrimal glands, salivary glands (parotid glands and submandibular glands), liver, spleen, kidneys, intestines and urinary bladder. Organs with the highest absorbed doses were the salivary glands, followed by the kidneys, receiving 1.24±0.26 and 0.99±0.31 mGy/MBq, respectively. The mean absorbed doses to the liver, urinary bladder and red marrow were 0.36±0.10, 0.243±0.09 and 0.048±0.05 mGy/MBq, respectively. The mean whole-body dose was 0.016±0.003 mGy/MBq.

CONCLUSION

Lu-DKFZ-PSMA-617 therapy is a safe option in the treatment of mCRPC patients.

PET imaging of 64Cu-DOTA-scFv-anti-PSMA lipid nanoparticles (LNPs): Enhanced tumor targeting over anti-PSMA scFv or untargeted LNPs

INTRODUCTION

Single chain (scFv) antibodies are ideal targeting ligands due to their modular structure, high antigen specificity and affinity. These monovalent ligands display rapid tumor targeting but have limitations due to their fast urinary clearance.

METHODS

An anti-prostate membrane antigen (PSMA) scFv with a site-specific cysteine was expressed and evaluated in a prostate cancer xenograft model by Cu-64 PET imaging. To enhance tumor accumulation, the scFv-cys was conjugated to the co-polymer DSPE-PEG-maleimide that spontaneously assembled into a homogeneous multivalent lipid nanoparticle (LNP).

RESULTS

The targeted LNP exhibited a 2-fold increase in tumor uptake compared to the scFv alone using two different thiol ester chemistries. The anti-PSMA scFv-LNP exhibited a 1.6 fold increase in tumor targeting over the untargeted LNP.

CONCLUSIONS

The targeted anti-PSMA scFv-LNP showed enhanced tumor accumulation over the scFv alone or the untargeted DOTA-micelle providing evidence for the development of this system for drug delivery.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Anti-tumor scFv antibody fragments have not achieved their therapeutic potential due to their fast blood clearance. Conjugation to an LNP enables multivalency to the tumor antigen as well as increased molecular size for chemotherapy drug delivery.

In Vivo 3T Magnetic Resonance Imaging Using a Biologically Specific Contrast Agent for Prostate Cancer: A Nude Mouse Model

We characterized in vivo a functional superparamagnetic iron-oxide magnetic resonance contrast agent that shortens the T2 relaxation time in magnetic resonance imaging (MRI) of prostate cancer xenografts. The agent was developed by conjugating Molday ION™ carboxyl-6 (MIC6), with a deimmunized mouse monoclonal antibody (muJ591) targeting prostate-specific membrane antigen (PSMA). This functional contrast agent could be used as a noninvasive method to detect prostate cancer cells that are PSMA positive and more readily differentiate them from surrounding tissues for treatment. The functional contrast agent was injected intravenously into mice and its effect was compared to both MIC6 (without conjugated antibody) and phosphate-buffered saline (PBS) injection controls. MR imaging was performed on a clinical 3T MRI scanner using a multiecho spin echo (MESE) sequence to obtain T2 relaxation time values. Inductively coupled plasma atomic emission spectroscopy was used to confirm an increase in elemental iron in injected mice tumours relative to controls. Histological examination of H&E stained tissues showed normal morphology of the tissues collected.

PSMA Expression in Papillary Thyroid Carcinoma: Opening a New Horizon in Management of Thyroid Cancer?

Prostate-specific membrane antigen (PSMA) is a type 2 transmembrane protein highly expressed in prostate cancer cells. We present the case of a 50-year-old man with metastatic papillary carcinoma of the thyroid, with rising thyroglobulin level and negative whole-body radioiodine scan after total thyroidectomy. Considering the limited treatment options available, it was decided to perform Ga-PSMA-HBED-CC PET/CT scan. It revealed intense radiotracer uptake in mediastinal and left supraclavicular lymph nodes, brain metastases, bilateral lung nodules, and skeletal sites. Patient also underwent F-FDG PET/CT. It demonstrated similar findings; however, the number of lesions detected in brain was less compared with Ga-PSMA PET/CT.

The therapeutic and diagnostic potential of the prostate specific membrane antigen/glutamate carboxypeptidase II (PSMA/GCPII) in cancer and neurological disease

Prostate specific membrane antigen (PSMA) otherwise known as glutamate carboxypeptidase II (GCPII) is a membrane bound protein that is highly expressed in prostate cancer and in the neovasculature of a wide variety of tumours including glioblastomas, breast and bladder cancers. This protein is also involved in a variety of neurological diseases including schizophrenia and ALS. In recent years, there has been a surge in the development of both diagnostics and therapeutics that take advantage of the expression and activity of PSMA/GCPII. These include gene therapy, immunotherapy, chemotherapy and radiotherapy. In this review, we discuss the biological roles that PSMA/GCPII plays, both in normal and diseased tissues, and the current therapies exploiting its activity that are at the preclinical stage. We conclude by giving an expert opinion on the future direction of PSMA/GCPII based therapies and diagnostics and hurdles that need to be overcome to make them effective and viable.

Radioimmunotherapy for Prostate Cancer--Current Status and Future Possibilities

Prostate cancer (PCa) is one of the most common cancers in men and is the second leading cause of cancer-related deaths in the USA. In the United States, it is the second most frequently diagnosed cancer after skin cancer, and in Europe it is number one. According to the American Cancer Society, approximately 221,000 men in the United States would be diagnosed with PCa during 2015, and approximately 28,000 would die of the disease. According to the International Agency for Research on Cancer, approximately 345,000 men were diagnosed with PCa in Europe during 2012, and despite more emphasis placed on early detection through routine screening, 72,000 men died of the disease. Hence, the need for improved therapy modalities is of utmost importance. And targeted therapies based on radiolabeled specific antibodies or peptides are a very interesting and promising alternative to increase the therapeutic efficacy and overall chance of survival of these patients. There are currently several preclinical and some clinical studies that have been conducted, or are ongoing, to investigate the therapeutic efficacy and toxicity of radioimmunotherapy (RIT) against PCa. One thing that is lacking in a lot of these published studies is the dosimetry data, which are needed to compare results between the studies and the study locations. Given the complicated tumor microenvironment and overall complexity of RIT to PCa, old and new targets and targeting strategies like combination RIT and pretargeting RIT are being improved and assessed along with various therapeutic radionuclides candidates. Given alone or in combination with other therapies, these new and improved strategies and RIT tools further enhance the clinical response to RIT drugs in PCa, making RIT for PCa an increasingly practical clinical tool.

New Strategies in Prostate Cancer: Prostate-Specific Membrane Antigen (PSMA) Ligands for Diagnosis and Therapy

Key issues for prostate cancer patients are the detection of recurrent disease and the treatment of metastasized cancer. Early detection is a major challenge for all conventional imaging modalities. Furthermore, therapy of patients with hormone-resistant tumor lesions presents a major clinical challenge. Because the prostate-specific membrane antigen (PSMA) is frequently overexpressed in prostate cancer, several PSMA-targeting molecules are under development to detect and treat metastatic castration-resistant prostate cancer (mCRPC). mCRPC represents a situation where cure is no longer achievable and novel therapeutic approaches for palliation and increase of survival are needed. In this article, we discuss the recent development for noninvasive detection of recurrent disease and therapy of mCRPC with corresponding PSMA-targeted radioligands.

Applications of indocyanine green in robotic urology

Indocyanine green is a fluorescent molecule with wide ranging applications in minimally invasive urological surgery. This article explores the utility of ICG assisted intraoperative fluorescence in robotic urology.

PSMA PET and Radionuclide Therapy in Prostate Cancer

Prostate cancer (PCa) is the most common malignancy in men and a major cause of cancer death. Accurate imaging plays an important role in diagnosis, staging, restaging, detection of biochemical recurrence, and for therapy of patients with PCa. Because no effective treatment is available for advanced PCa, there is an urgent need to develop new and more effective therapeutic strategies. To optimize treatment outcome, especially in high-risk patients with PCa, therapy for PCa is moving rapidly toward personalization. Medical imaging, including positron emission tomography (PET)/computed tomography (CT), plays an important role in personalized medicine in oncology. In the recent years, much focus has been on prostate-specific membrane antigen (PSMA) as a promising target for imaging and therapy with radionuclides, as it is upregulated in most PCa. In the prostate, one potential role for PSMA PET imaging is to help guide focal therapy. Several studies have shown great potential of PSMA PET/CT for initial staging, lymph node staging, and detection of recurrence of PCa, even at very low prostate-specific antigen values after primary therapy. Furthermore, studies have shown that PSMA PET/CT has a higher detection rate than choline PET/CT. Radiolabeled PSMA ligands for therapy show promise in several studies with metastatic PCa and is an area of active investigation. The "image and treat" strategy, with radiolabeled PSMA ligands, has the potential to improve the treatment outcome of patients with PCa and is paving the way for precision medicine in PCa. The aim of this review is to give an overview of recent advancement in PSMA PET and radionuclide therapy for PCa.

First-in-Human Imaging with 89Zr-Df-IAB2M Anti-PSMA Minibody in Patients with Metastatic Prostate Cancer: Pharmacokinetics, Biodistribution, Dosimetry, and Lesion Uptake

We conducted a phase I dose-escalation study with ⁸⁹Zr-desferrioxamine-IAB2M (⁸⁹Zr-IAB2M), an anti-prostate-specific membrane antigen minibody, in patients with metastatic prostate cancer.

METHODS

Patients received 185 MBq (5 mCi) of ⁸⁹Zr-IAB2M and Df-IAB2M at total mass doses of 10 (n = 6), 20 (n = 6), and 50 mg (n = 6). Whole-body and serum clearance, normal-organ and lesion uptake, and radiation absorbed dose were estimated, and the effect of mass escalation was analyzed.

RESULTS

Eighteen patients were injected and scanned without side effects. Whole-body clearance was monoexponential, with a median biologic half-life of 215 h, whereas serum clearance showed biexponential kinetics, with a median biologic half-life of 3.7 (12.3%/L) and 33.8 h (17.9%/L). The radiation absorbed dose estimates were 1.67, 1.36, and 0.32 mGy/MBq to liver, kidney, and marrow, respectively, with an effective dose of 0.41 mSv/MBq (1.5 rem/mCi). Both skeletal and nodal lesions were detected with ⁸⁹Zr-IAB2M, most visualized by 48-h imaging.

CONCLUSION

⁸⁹Zr-IAB2M is safe and demonstrates favorable biodistribution and kinetics for targeting metastatic prostate cancer. Imaging with 10 mg of minibody mass provides optimal biodistribution, and imaging at 48 h after injection provides good lesion visualization. Assessment of lesion targeting is being studied in detail in an expansion cohort.

MOR209/ES414, a Novel Bispecific Antibody Targeting PSMA for the Treatment of Metastatic Castration-Resistant Prostate Cancer

Treatment of metastatic, castration-resistant prostate cancer (mCRPC) remains a highly unmet medical need and current therapies ultimately result in disease progression. Immunotherapy is a rapidly growing approach for treatment of cancer but has shown limited success to date in the treatment of mCRPC. We have developed a novel humanized bispecific antibody, MOR209/ES414, built on the ADAPTIR (modular protein technology) platform, to redirect T-cell cytotoxicity toward prostate cancer cells by specifically targeting T cells through CD3ε to prostate cancer cells expressing PSMA (prostate-specific membrane antigen). In vitro cross-linking of T cells with PSMA-expressing tumor cells by MOR209/ES414 triggered potent target-dependent tumor lysis and induction of target-dependent T-cell activation and proliferation. This activity occurred at low picomolar concentrations of MOR209/ES414 and was effective at low T-effector to tumor target cell ratios. In addition, cytotoxic activity was equivalent over a wide range of PSMA expression on target cells, suggesting that as few as 3,700 PSMA receptors per cell are sufficient for tumor lysis. In addition to high sensitivity and in vitro activity, MOR209/ES414 induced limited production of cytokines compared with other bispecific antibody formats. Pharmacokinetic analysis of MOR209/ES414 demonstrated a serum elimination half-life in NOD/SCID γ (NSG) mice of 4 days. Administration of MOR209/ES414 in murine xenograft models of human prostate cancer significantly inhibited tumor growth, prolonged survival, and decreased serum prostate-specific antigen levels only in the presence of adoptively transferred human T cells. On the basis of these preclinical findings, MOR209/ES414 warrants further investigation as a potential therapeutic for the treatment of CRPC. Mol Cancer Ther; 15(9); 2155-65. ©2016 AACR.

Progress in SPECT/CT Imaging of Prostate Cancer

Prostate cancer is the most common type of cancer (other than skin cancer) among men in the United States. Although prostate cancer is one of the few cancers that grow so slowly that it may never threaten the lives of some patients, it can be lethal once metastasized. Indium-111 capromab pendetide (ProstaScint®, Cytogen Corporation, Princeton, NJ) imaging is indicated for staging and recurrence detection of the disease, and is particularly useful to determine whether or not the disease has spread to distant metastatic sites. However, the interpretation of 111 In-capromab pendetide is challenging without correlated structural information mostly because the radiopharmaceutical demonstrates nonspecific uptake in the normal vasculature, bowel, bone marrow, and the prostate gland. We developed an improved method of imaging and localizing 111 In-Capromab pendetide using a SPECT/CT imaging system. The specific goals included: i) development and application of a novel iterative SPECT reconstruction algorithm that utilizes a priori information from coregistered CT; and ii) assessment of clinical impact of adding SPECT/CT for prostate cancer imaging with capromab pendetide utilizing the standard and novel reconstruction techniques.

Patient imaging studies with capromab pendetide were performed from 1999 to 2004 using two different SPECT/CT scanners, a prototype SPECT/CT system and a commercial SPECT/CT system (Discovery VH, GE Healthcare, Waukesha, WI). SPECT projection data from both systems were reconstructed using an experimental iterative algorithm that compensates for both photon attenuation and collimator blurring. In addition, the data obtained from the commercial system were reconstructed with attenuation correction using an OSEM reconstruction supplied by the camera manufacturer for routine clinical interpretation. For 12 sets of patient data, SPECT images reconstructed using the experimental algorithm were interpreted separately and compared with interpretation of images obtained using the standard reconstruction technique. The experimental reconstruction algorithm improved spatial resolution, reduced streak artifacts, and yielded a better correlation with anatomic details of CT in comparison to conventional reconstruction methods ( e.g., filtered back-projection or OSEM with attenuation correction only). Images produced with the experimental algorithm produced a subjective improvement in the confidence of interpretation for 11 of 12 studies. There were also changes in interpretations for 4 of 12 studies although the changes were not sufficient to alter prognosis or the patient treatment plan.

Theranostic Agents for Photodynamic Therapy of Prostate Cancer by Targeting Prostate-Specific Membrane Antigen

Prostatectomy has been the mainstay treatment for men with localized prostate cancer. Surgery, however, often can result in major side effects, which are caused from damage and removal of nerves and muscles surrounding the prostate. A technology that can help surgeons more precisely identify and remove prostate cancer resulting in a more complete prostatectomy is needed. Prostate-specific membrane antigen (PSMA), a type II membrane antigen highly expressed in prostate cancer, has been an attractive target for imaging and therapy. The objective of this study is to develop low molecular weight PSMA-targeted photodynamic therapy (PDT) agents, which would provide image guidance for prostate tumor resection and allow for subsequent PDT to eliminate unresectable or remaining cancer cells. On the basis of our highly negatively charged, urea-based PSMA ligand PSMA-1, we synthesized two PSMA-targeting PDT conjugates named PSMA-1-Pc413 and PSMA-1-IR700. In in vitro cellular uptake experiments and in vivo animal imaging experiments, the two conjugates demonstrated selective and specific uptake in PSMA-positive PC3pip cells/tumors, but not in PSMA-negative PC3flu cells/tumors. Further in vivo photodynamic treatment proved that the two PSMA-1-PDT conjugates can effectively inhibit PC3pip tumor progression. The two PSMA-1-PDT conjugates reported here may have the potential to aid in the detection and resection of prostate cancers. It may also allow for the identification of unresectable cancer tissue and PDT ablation of such tissue after surgical resection with potentially less damage to surrounding tissues. Mol Cancer Ther; 15(8); 1834-44. ©2016 AACR.

PET imaging of prostate-specific membrane antigen in prostate cancer: current state of the art and future challenges

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a cell surface enzyme that is highly expressed in prostate cancer (PCa) and is currently being extensively explored as a promising target for molecular imaging in a variety of clinical contexts. Novel antibody and small-molecule PSMA radiotracers labeled with a variety of radionuclides for positron emission tomography (PET) imaging applications have been developed and explored in recent studies.

METHODS

A great deal of progress has been made in defining the clinical utility of this class of PET agents through predominantly small and retrospective clinical studies. The most compelling data to date has been in the setting of biochemically recurrent PCa, where PSMA-targeted radiotracers have been found to be superior to conventional imaging and other molecular imaging agents for the detection of locally recurrent and metastatic PCa.

RESULTS

Early data, however, suggest that initial lymph node staging before definitive therapy in high-risk primary PCa patients may be limited, although intraoperative guidance may still hold promise. Other examples of potential promising applications for PSMA PET imaging include non-invasive characterization of primary PCa, staging and treatment planning for PSMA-targeted radiotherapeutics, and guidance of focal therapy for oligometastatic disease.

CONCLUSIONS

However, all of these indications and applications for PCa PSMA PET imaging are still lacking and require large, prospective, systematic clinical trials for validation. Such validation trials are needed and hopefully will be forthcoming as the fields of molecular imaging, urology, radiation oncology and medical oncology continue to define and refine the utility of PSMA-targeted PET imaging to improve the management of PCa patients.

Characterization of a novel metastatic prostate cancer cell line of LNCaP origin

BACKGROUND

The LNCaP cell line was originally isolated from the lymph node of a patient with metastatic prostate cancer. Many cell lines have been derived from LNCaP by selective pressures to study different aspects of prostate cancer progression. When injected subcutaneously into male athymic nude mice, LNCaP and its derivatives rarely metastasize.

METHODS

Here, we describe the characteristics of a new LNCaP derivative, JHU-LNCaP-SM, which was generated by long term passage in normal cell culture conditions.

RESULTS

Short tandem repeat (STR) analysis and genomic sequencing verified JHU-LNCaP-SM derivation from parental LNCaP cells. JHU-LNCaP-SM cells express the same mutated androgen receptor (AR) but unlike LNCaP, are no longer androgen dependent for growth. The cells demonstrate an attenuated androgen responsiveness in transcriptional assays and retain androgen sensitive expression of PSA, AR, and PSMA. Unlike parental LNCaP, JHU-LNCaP-SM cells quickly form subcutaneous tumors in male athymic nude mice, reliably metastasize to the lymph nodes and display a striking intra-tumoral and spreading hemorrhagic phenotype as tumor xenografts.

CONCLUSIONS

The JHU-LNCaP-SM cell line is a new isolate of LNCaP, which facilitates practical, preclinical studies of spontaneous metastasis of prostate cancer through lymphatic tissues.

Quantitative [Fe]MRI of PSMA-targeted SPIONs specifically discriminates among prostate tumor cell types based on their PSMA expression levels

We report the development, experimental verification, and application of a general theory called [Fe]MRI (pronounced fem-ree) for the non-invasive, quantitative molecular magnetic resonance imaging (MRI) of added magnetic nanoparticles or other magnetic contrast agents in biological tissues and other sites. [Fe]MRI can easily be implemented on any MRI instrument, requiring only measurements of the background nuclear magnetic relaxation times (T₁, T₂) of the tissue of interest, injection of the magnetic particles, and the subsequent acquisition of a pair of T₁-weighted and T₂-weighted images. These images, converted into contrast images, are subtracted to yield a contrast difference image proportional to the absolute nanoparticle, iron concentration, ([Fe]) image. [Fe]MRI was validated with the samples of superparamagnetic iron oxide nanoparticles (SPIONs) both in agarose gels and bound to human prostate tumor cells. The [Fe]MRI measurement of the binding of anti-prostate specific membrane antigen (PSMA) conjugated SPIONs to PSMA-positive LNCaP and PSMA-negative DU145 cells in vitro allowed a facile discrimination among prostate tumor cell types based on their PSMA expression level. The low [Fe] detection limit of ~2 μM for SPIONs allows sensitive MRI of added iron at concentrations considerably below the US Food and Drug Administration’s human iron dosage guidelines (<90 μM, 5 mg/kg).

Antibody-mediated delivery of therapeutics for cancer therapy

INTRODUCTION

Antibody-conjugated therapies (ACTs) combine the specificity of monoclonal antibodies to target cancer cells directly with highly potent payloads, often resulting in superior efficacy and/or reduced toxicity. This represents a new approach to the treatment of cancer. There have been highly promising clinical trial results using this approach with improvements in linker and payload technology. The breadth of current trials examining ACTs in haematological malignancies and solid tumours indicate the potential for clinical impact.

AREAS COVERED

This review will provide an overview of ACTs currently in clinical development as well as the principles of antibody delivery and types of payloads used, including cytotoxic drugs, radiolabelled isotopes, nanoparticle-based siRNA particles and immunotoxins.

EXPERT OPINION

The focus of much of the clinical activity in ACTs has, understandably, been on their use as a monotherapy or in combination with standard of care drugs. This will continue, as will the search for better targets, linkers and payloads. Increasingly, as these drugs enter routine clinical care, important questions will arise regarding how to optimise ACT treatment approaches, including investigation of resistance mechanisms, biomarker and patient selection strategies, understanding of the unique toxicities of these drugs, and combinatorial approaches with standard therapies as well as emerging therapeutic agents like immunotherapy.

[(18)F]Fluorobenzoyllysinepentanedioic Acid Carbamates: New Scaffolds for Positron Emission Tomography (PET) Imaging of Prostate-Specific Membrane Antigen (PSMA)

Radiolabeled urea-based low-molecular weight inhibitors of the prostate-specific membrane antigen (PSMA) are under intense investigation as imaging and therapeutic agents for prostate and other cancers. In an effort to provide agents with less nontarget organ uptake than the ureas, we synthesized four (18)F-labeled inhibitors of PSMA based on carbamate scaffolds. 4-Bromo-2-[(18)F]fluorobenzoyllysineoxypentanedioic acid (OPA) carbamate [(18)F]23 and 4-iodo-2-[(18)F]fluorobenzoyllysine OPA carbamate [(18)F]24 in particular exhibited high target-selective uptake in PSMA+ PC3 PIP tumor xenografts, with tumor-to-kidney ratios of >1 by 4 h postinjection, an important benchmark. Because of its high tumor uptake (90% injected dose per gram of tissue at 2 h postinjection) and high tumor-to-organ ratios, [(18)F]23 is promising for clinical translation. Prolonged tumor-specific uptake demonstrated by [(18)F]24, which did not reach equilibrium during the 4 h study period, suggests carbamates as alternative scaffolds for mitigating dose to nontarget tissues.

The clinical and therapeutic uses of MDM2 and PSMA and their potential interaction in aggressive cancers

Prostate-specific membrane antigen (PSMA) overexpression is observed in the neovasculature of solid tumors, but not in the vasculature of normal tissues. Increased PSMA expression is positively associated with tumor stage and grade, although its function in cancer remains unclear. Mouse double minute 2 (MDM2) is a negative regulator of the p53 tumor suppressor and is reported to regulate VEGF expression and angiogenesis. Both proteins have been considered as biomarkers and therapeutic targets for advanced solid tumors. Our work and a recent microarray-based gene profiling study suggest there could be signaling interplay between MDM2 and PSMA. We herein review the mechanisms underlining the outgrowth of tumors associated with PSMA and MDM2, their potential interaction and how this may be applied to anticancer therapeutics.

Single-Dose Anti-CD138 Radioimmunotherapy: Bismuth-213 is More Efficient than Lutetium-177 for Treatment of Multiple Myeloma in a Preclinical Model

Objectives

Radioimmunotherapy (RIT) has emerged as a potential treatment option for multiple myeloma (MM). In humans, a dosimetry study recently showed the relevance of RIT using an antibody targeting the CD138 antigen. The therapeutic efficacy of RIT using an anti-CD138 antibody coupled to ²¹³Bi, an α-emitter, was also demonstrated in a preclinical MM model. Since then, RIT with β-emitters has shown efficacy in treating hematologic cancer. In this paper, we investigate the therapeutic efficacy of RIT in the 5T33 murine MM model using a new anti-CD138 monoclonal antibody labeled either with ²¹³Bi for α-RIT or ¹⁷⁷Lu for β-RIT.

Methods

A new monoclonal anti-CD138 antibody, 9E7.4, was generated by immunizing a rat with a murine CD138-derived peptide. Antibody specificity was validated by flow cytometry, biodistribution, and α-RIT studies. Then, a β-RIT dose-escalation assay with the ¹⁷⁷Lu-radiolabeled 9E7.4 mAb was performed in KalwRij C57/BL6 mice 10 days after i.v. engraftment with 5T33 MM cells. Animal survival and toxicological parameters were assessed to define the optimal activity.

Results

α-RIT performed with 3.7 MBq of ²¹³Bi-labeled 9E7.4 anti-CD138 mAb increased median survival to 80 days compared to 37 days for the untreated control and effected cure in 45% of animals. β-RIT performed with 18.5 MBq of ¹⁷⁷Lu-labeled 9E7.4 mAb was well tolerated and significantly increased mouse survival (54 vs. 37 days in the control group); however, no mice were cured with this treatment.

Conclusion

This study revealed the advantages of α-RIT in the treatment of MM in a preclinical model where β-RIT shows almost no efficacy.

PSMA Ligands for Radionuclide Imaging and Therapy of Prostate Cancer: Clinical Status

Prostate cancer (PCa) is the most common malignancy in men worldwide, leading to substantial morbidity and mortality. At present, imaging of PCa has become increasingly important for staging, restaging, and treatment selection. Until recently, choline-based positron emission tomography/computed tomography (PET/CT) represented the state-of-the-art radionuclide imaging technique for these purposes. However, its application is limited to patients with high PSA levels and Gleason scores. Prostate-specific membrane antigen (PSMA) is a promising new target for specific imaging of PCa, because it is upregulated in the majority of PCa. Moreover, PSMA can serve as a target for therapeutic applications. Currently, several small-molecule PSMA ligands with excellent in vivo tumor targeting characteristics are being investigated for their potential in theranostic applications in PCa. Here, a review of the recent developments in PSMA-based diagnostic imaging and therapy in patients with PCa with radiolabeled PSMA ligands is provided.

Ipilimumab in the treatment of prostate cancer

Ipilimumab (Yervoy(®); Bristol-Myers Squibb, NY, USA) is a fully human monoclonal antibody targeting CTLA-4 and is approved for the treatment of metastatic melanoma. Preclinical and clinical studies have shown its activity in a number of different cancer types, including prostate cancer. Recently, the results from a Phase III study of ipilimumab in prostate cancer patients with prior docetaxel therapy were reported. Although the study did not meet the primary end point of improved overall survival, prespecified subset analyses suggested that ipilimumab may be more active in men with lower disease burden, which suggests that immunotherapy should be tested early in men with castration-refractory prostate cancer. Immune-related adverse events are common and most can be well managed with standard immunosuppressive algorithms.

Chimeric antigen receptor-engineered T cells for the treatment of metastatic prostate cancer

Cancer immunotherapy was selected as the Breakthrough of the Year 2013 by the editors of Science, in part because of the successful treatment of refractory hematological malignancies with adoptive transfer of chimeric antigen receptor (CAR)-engineered T cells. Effective treatment of B cell leukemia may pave the road to future treatment of solid tumors, using similar approaches. The prostate expresses many unique proteins and, since the prostate gland is a dispensable organ, CAR T cells can potentially be used to target these tissue-specific antigens. However, the location and composition of prostate cancer metastases complicate the task of treating these tumors. It is therefore likely that more sophisticated CAR T cell approaches are going to be required for prostate metastasis than for B cell malignancies. Two main challenges that need to be resolved are how to increase the migration and infiltration of CAR T cells into prostate cancer bone metastases and how to counteract the immunosuppressive microenvironment found in bone lesions. Inclusion of homing (chemokine) receptors in CAR T cells may improve their recruitment to bone metastases, as may antibody-based combination therapies to normalize the tumor vasculature. Optimal activation of CAR T cells through the introduction of multiple costimulatory domains would help to overcome inhibitory signals from the tumor microenvironment. Likewise, combination therapy with checkpoint inhibitors that can reduce tumor immunosuppression may help improve efficacy. Other elegant approaches such as induced expression of immune stimulatory cytokines upon target recognition may also help to recruit other effector immune cells to metastatic sites. Although toxicities are difficult to predict in prostate cancer, severe on-target/off-tumor toxicities have been observed in clinical trials with use of CAR T cells against hematological malignancies; therefore, the choice of the target antigen is going to be crucial. This review focuses on different means of accomplishing maximal effectiveness of CAR T cell therapy for prostate cancer bone metastases while minimizing side effects and CAR T cell-associated toxicities. CAR T cell-based therapies for prostate cancer have the potential to be a therapy model for other solid tumors.

Next revolution in molecular theranostics: personalized medicine for urologic cancers

Extensive lists of molecular biomarkers are currently evaluated as potential targets for directed cancer therapies. We reviewed three potential candidate biomarkers to play a role in the near future as molecular theranostics for urologic malignancies. Carbonic anhydrase type IX is a surrogate marker of hypoxia highly expressed in cancer cells. Their expression and clinical significance in kidney and urothelial bladder cancer are discussed as well as the main therapeutic approaches that are currently under evaluation. For prostate cancer, available evidence on the use of prostate-specific membrane antigen and neuropeptide receptors radiolabeled analog and the undergoing clinical studies are also analyzed and discussed at different stages of prostate cancer.

Evaluation of DOTA-chelated neurotensin analogs with spacer-enhanced biological performance for neurotensin-receptor-1-positive tumor targeting

INTRODUCTION

Neurotensin receptor 1 (NTR1) is overexpressed in many cancer types. Neurotensin (NT), a 13 amino acid peptide, is the native ligand for NTR1 and exhibits high (nM) affinity to the receptor. Many laboratories have been investigating the development of diagnostic and therapeutic radiopharmaceuticals for NTR1-positive cancers based on the NT peptide. To improve the biological performance for targeting NTR1, we proposed NT analogs with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelation system and different lengths of spacers.

METHODS

We synthesized four NTR1-targeted conjugates with spacer lengths from 0 to 9 atoms (null (N0), β-Ala-OH (N1), 5-Ava-OH (N2), and 8-Aoc-OH (N3)) between the DOTA and the pharmacophore. In vitro competitive binding, internalization and efflux studies were performed on all four NT analogs. Based on these findings, metabolism studies were carried out on our best performing conjugate, (177)Lu-N1. Lastly, in vivo biodistribution and SPECT/CT imaging studies were performed using (177)Lu-N1 in an HT-29 xenograft mouse model.

RESULTS

As shown in the competitive binding assays, the NT analogs with different spacers (N1, N2 and N3) exhibited lower IC50 values than the NT analog without a spacer (N0). Furthermore, N1 revealed higher retention in HT-29 cells with more rapid internalization and slower efflux than the other NT analogs. In vivo biodistribution and SPECT/CT imaging studies of (177)Lu-N1 demonstrated excellent accumulation (3.1 ± 0.4%ID/g) in the NTR1-positive tumors at 4h post-administration.

CONCLUSIONS

The DOTA chelation system demonstrated some modest steric inhibition of the pharmacophore. However, the insertion of a 4-atom hydrocarbon spacer group restored optimal binding affinity of the analog. The in vivo assays indicated that (177)Lu-N1 could be used for imaging and radiotherapy of NTR1-positive tumors.

Early side effects and first results of radioligand therapy with (177)Lu-DKFZ-617 PSMA of castrate-resistant metastatic prostate cancer: a two-centre study

Background

Radioligand therapy (RLT) with ¹⁷⁷Lu-DKFZ-617 PSMA (Lu-PSMA) (prostate-specific membrane antigen) is a novel targeted therapy of metastatic prostate cancer. We analysed retrospectively the early side effects and the response rate in the first patients, who received a therapy with Lu-PSMA in our departments.

Methods

RLT was performed in ten hormone- and/or chemo-refractory patients with distant metastases and progressive disease (mean age 73.5 years). ⁶⁸Ga-PSMA HBED-CC PET/CT was performed in all patients prior to RLT. The median PSA level prior to the therapy was 298.5 ng/ml (range 5–853 ng/ml). All patients received CBC, renal and liver function tests the day before and 2 days after application (mean administered activity 5.6 GBq, range 4.1–6.1 GBq), followed by further tests every 2 weeks. All patients were contacted by telephone every week regarding side effects or any positive and negative changes.

Results

Eight weeks after the therapy, seven patients (70 %) experienced a PSA decline, of whom six experienced more than 30 % and five more than 50 %. Three patients showed a progressive disease according to the PSA increase. No patient experienced any side effects immediately after injection of Lu-PSMA. Relevant hematotoxicity (grade 3 or 4) occurred 7 weeks after the administration in just one patient. The same patient showed a leucopenia grade 2. Two patients showed a disturbance of only 1 hematologic cell line, whereas one patient showed a reduction of grades 1 and 2 in leucocytes and thrombocytes, respectively. Six patients did not show any hematotoxicity during the 8 weeks after therapy. There was no relevant nephrotoxicity (grade 3 or 4).

Conclusions

Our initial results indicate that RLT with Lu-PSMA is safe and seems to have low early side-effect profile. A relevant PSA decline was detected in 70 % of patients.

A high-affinity [(18)F]-labeled phosphoramidate peptidomimetic PSMA-targeted inhibitor for PET imaging of prostate cancer

INTRODUCTION

In this study, a structurally modified phosphoramidate scaffold, with improved prostate-specific membrane antigen (PSMA) avidity, stability and in vivo characteristics, as a PET imaging agent for prostate cancer (PCa), was prepared and evaluated.

METHODS

p-Fluorobenzoyl-aminohexanoate and 2-(3-hydroxypropyl)glycine were introduced into the PSMA-targeting scaffold yielding phosphoramidate 5. X-ray crystallography was performed on the PSMA/5 complex. [(18)F]5 was synthesized, and cell uptake and internalization studies were conducted in PSMA(+) LNCaP and CWR22Rv1 cells and PSMA(-) PC-3 cells. In vivo PET imaging and biodistribution studies were performed at 1 and 4 h post injection in mice bearing CWR22Rv1 tumor, with or without blocking agent.

RESULTS

The crystallographic data showed interaction of the p-fluorobenzoyl group with an arene-binding cleft on the PSMA surface. In vitro studies revealed elevated uptake of [(18)F]5 in PSMA(+) cells (2.2% in CWR22Rv1 and 12.1% in LNCaP) compared to PSMA(-) cells (0.08%) at 4 h. In vivo tumor uptake of 2.33% ID/g and tumor-to-blood ratio of 265:1 was observed at 4 h.

CONCLUSIONS

We have successfully synthesized, radiolabeled and evaluated a new PSMA-targeted PET agent. The crystal structure of the PSMA/5 complex highlighted the interactions within the arene-binding cleft contributing to the overall complex stability. The high target uptake and rapid non-target clearance exhibited by [(18)F]5 in PSMA(+) xenografts substantiates its potential use for PET imaging of PCa.

ADVANCES IN KNOWLEDGE

The only FDA-approved imaging agent for PCa, Prostascint®, targets PSMA but suffers from inherent shortcomings. The data acquired in this manuscript confirmed that our new generation of [(18)F]-labeled PSMA inhibitor exhibited promising in vivo performance as a PET imaging agent for PCa and is well-positioned for subsequent clinical trials. Implications for Patient Care Our preliminary data demonstrate that this tracer possesses the required imaging characteristics to be sensitive and specific for PCa imaging in patients at all stages of the disease.

Prostate cancer radiotherapy: potential applications of metal nanoparticles for imaging and therapy

Prostate cancer (CaP) is the most commonly diagnosed cancer in males. There have been dramatic technical advances in radiotherapy delivery, enabling higher doses of radiotherapy to primary cancer, involved lymph nodes and oligometastases with acceptable normal tissue toxicity. Despite this, many patients relapse following primary radical therapy, and novel treatment approaches are required. Metal nanoparticles are agents that promise to improve diagnostic imaging and image-guided radiotherapy and to selectively enhance radiotherapy effectiveness in CaP. We summarize current radiotherapy treatment approaches for CaP and consider pre-clinical and clinical evidence for metal nanoparticles in this condition.

Targeting tumor vasculature with aptamer-functionalized doxorubicin-polylactide nanoconjugates for enhanced cancer therapy

An A10 aptamer (Apt)-functionalized, sub-100 nm doxorubicin-polylactide (Doxo-PLA) nanoconjugate (NC) with controlled release profile was developed as an intravenous therapeutic strategy to effectively target and cytoreduce canine hemangiosarcoma (cHSA), a naturally occurring solid tumor malignancy composed solely of tumor-associated endothelium. cHSA consists of a pure population of malignant endothelial cells expressing prostate-specific membrane antigen (PSMA) and is an ideal comparative tumor model system for evaluating the specificity and feasibility of tumor-associated endothelial cell targeting by A10 Apt-functionalized NC (A10 NC). In vitro, A10 NCs were selectively internalized across a panel of PSMA-expressing cancer cell lines, and when incorporating Doxo, A10 Doxo-PLA NCs exerted greater cytotoxic effects compared to nonfunctionalized Doxo-PLA NCs and free Doxo. Importantly, intravenously delivered A10 NCs selectively targeted PSMA-expressing tumor-associated endothelial cells at a cellular level in tumor-bearing mice and dramatically increased the uptake of NCs by endothelial cells within the local tumor microenvironment. By virtue of controlled drug release kinetics and selective tumor-associated endothelial cell targeting, A10 Doxo-PLA NCs possess a desirable safety profile in vivo, being well-tolerated following high-dose intravenous infusion in mice, as supported by the absence of any histologic organ toxicity. In cHSA-implanted mice, two consecutive intravenous infusions of A10 Doxo-PLA NCs exerted rapid and substantial cytoreductive activities within a period of 7 days, resulting in greater than 70% reduction in macroscopic tumor-associated endothelial cell burden as a consequence of enhanced cell death and necrosis.

Synthesis and evaluation of constrained phosphoramidate inhibitors of prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a cell-surface enzyme-biomarker that is actively pursued for targeted delivery of imaging and therapeutic agents for prostate cancer. Our lab has developed PSMA inhibitors based on a phosphoramidate scaffold, which has shown both high selectivity for PSMA-positive tumors and rapid clearance in vivo when radiolabeled with (18)F. However, this scaffold exhibits hydrolytic instability under low pH and high temperature conditions, barring the use of other imaging or therapeutic radionuclides such as (68)Ga or (177)Lu. Previous studies in our lab have shown a trend in increasing acid stability as the distance between the phosphoramidate core and the α-carboxylate of the P1 residue is increased. Therefore, a new generation of phosphoramidate inhibitors was developed based on trans-4-hydroxyproline as the P1 residue to restrict the interaction of the α-carboxylate to the phosphoramidate core. These hydroxyproline inhibitors demonstrated comparable IC50 values to earlier generations as well as enhanced thermal and acid stability.

Immunotherapy for prostate cancer: recent developments and future challenges

Since the approval of sipuleucel-T for men with metastatic castrate resistant prostate cancer in 2010, great strides in the development of anti-cancer immunotherapies have been made. Current drug development in this area has focused primarily on antigen-specific (i.e. cancer vaccines and antibody based therapies) or checkpoint inhibitor therapies, with the checkpoint inhibitors perhaps gaining the most attention as of late. Indeed, drugs blocking the inhibitory signal generated by the engagement of cytotoxic T-lymphocyte antigen-4 (CTLA-4) and programmed cell death-1 (PD-1) found on T-cells has emerged as potent means to combat the immunosuppressive milieu. The anti-CTLA-4 monoclonal antibody ipilimumab has already been approved in advanced melanoma and two phase III trials evaluating ipilimumab in men with metastatic castrate-resistant prostate cancer are underway. A phase III trial evaluating ProstVac-VF, a poxvirus-based therapeutic prostate cancer vaccine, is also underway. While there has been reason for encouragement over the past few years, many questions regarding the use of immunotherapies remain. Namely, it is unclear what stage of disease is most likely to benefit from these approaches, how best to incorporate said treatments with each other and into our current treatment regimens and which therapy is most appropriate for which disease. Herein we review some of the recent advances in immunotherapy as related to the treatment of prostate cancer and outline some of the challenges that lie ahead.

Preclinical evaluation of 86Y-labeled inhibitors of prostate-specific membrane antigen for dosimetry estimates

(86)Y (half-life = 14.74 h, 33% β(+)) is within an emerging class of positron-emitting isotopes with relatively long physical half-lives that enables extended imaging of biologic processes. We report the synthesis and evaluation of 3 low-molecular-weight compounds labeled with (86)Y for imaging the prostate-specific membrane antigen (PSMA) using PET. Impetus for the study derives from the need to perform dosimetry estimates for the corresponding (90)Y-labeled radiotherapeutics.

METHODS

Multistep syntheses were used in preparing (86)Y- 4: - 6: PSMA inhibition constants were evaluated by competitive binding assay. In vivo characterization using tumor-bearing male mice was performed by PET/CT for (86)Y- 4: - 6: and by biodistribution studies of (86)Y- 4: and (86)Y- 6: out to 24 h after injection. Quantitative whole-body PET scans were recorded to measure the kinetics for 14 organs in a male baboon using (86)Y- 6 RESULTS: Compounds (86)Y- 4: - 6: were obtained in high radiochemical yield and purity, with specific radioactivities of more than 83.92 GBq/μmol. PET imaging and biodistribution studies using PSMA-positive PC-3 PIP and PSMA-negative PC-3 flu tumor-bearing mice revealed that (86)Y- 4-6: had high site-specific uptake in PSMA-positive PC-3 PIP tumor starting at 20 min after injection and remained high at 24 h. Compound (86)Y- 6: demonstrated the highest tumor uptake and retention, with 32.17 ± 7.99 and 15.79 ± 6.44 percentage injected dose per gram (%ID/g) at 5 and 24 h, respectively. Low activity concentrations were associated with blood and normal organs, except for the kidneys, a PSMA-expressing tissue. PET imaging in baboons reveals that all organs have a 2-phase (rapid and slow) clearance, with the highest uptake (8 %ID/g) in the kidneys at 25 min. The individual absolute uptake kinetics were used to calculate radiation doses using the OLINDA/EXM software. The highest mean absorbed dose was received by the renal cortex, with 1.9 mGy per MBq of (86)Y- 6:

CONCLUSION

Compound (86)Y- 6: is a promising candidate for quantitative PET imaging of PSMA-expressing tumors. Dosimetry calculations indicate promise for future (90)Y or other radiometals that could use a similar chelator/scaffold combination for radiopharmaceutical therapy based on the structure of 6.

Tumor immunotargeting using innovative radionuclides

This paper reviews some aspects and recent developments in the use of antibodies to target radionuclides for tumor imaging and therapy. While radiolabeled antibodies have been considered for many years in this context, only a few have reached the level of routine clinical use. However, alternative radionuclides, with more appropriate physical properties, such as lutetium-177 or copper-67, as well as alpha-emitting radionuclides, including astatine-211, bismuth-213, actinium-225, and others are currently reviving hopes in cancer treatments, both in hematological diseases and solid tumors. At the same time, PET imaging, with short-lived radionuclides, such as gallium-68, fluorine-18 or copper-64, or long half-life ones, particularly iodine-124 and zirconium-89 now offers new perspectives in immuno-specific phenotype tumor imaging. New antibody analogues and pretargeting strategies have also considerably improved the performances of tumor immunotargeting and completely renewed the interest in these approaches for imaging and therapy by providing theranostics, companion diagnostics and news tools to make personalized medicine a reality.

PSMA-targeting iron oxide magnetic nanoparticles enhance MRI of preclinical prostate cancer

Aim: To evaluate the potential of newly-developed, biocompatible iron oxide magnetic nanoparticles (MNPs) conjugated with J591, an antibody to an extracellular epitope of PSMA, to enhance MRI of prostate cancer. Materials & methods: Specific binding to PSMA by J591-MNP was investigated in vitro. MRI studies were performed on orthotopic tumor-bearing NOD.SCID mice 2 h and 24 h after intravenous injection of J591-MNPs, or non-targeting MNPs. Results & conclusion: In vitro, MNPs did not affect prostate cancer cell viability, and conjugation to J591 did not compromise antibody specificity and enhanced cellular iron uptake. Magnetic resonance contrast of tumors was increased in vivo using PSMA-targeting MNPs, but not by non-targeting MNPs. This provides proof-of-concept that PSMA-targeting MNPs have potential to enhance magnetic resonance detection/localization of prostate cancer.

Microtubule inhibitor-based antibody-drug conjugates for cancer therapy

The specificity of monoclonal antibodies represents a potential therapeutic advantage, but their use as single agents in oncology has proven limited to date. The development of antibody-drug conjugates (ADCs) takes advantage of the specificity of the monoclonal antibody and potent cytotoxic effect of chemotherapy, leading to enhanced cytotoxicity in target cells and limiting toxicity to normal tissue. Microtubules represent a validated oncologic target in a range of tumor types, with a number of anti-microtubule targeting cytotoxic drugs approved for cancer use. The systemic use of potent microtubule-binding agents is limited by their effects in normal cells, which leads to toxicity including myelosuppression and peripheral neuropathy. Linking these agents to monoclonal antibodies may limit toxicity to normal tissues and increase drug concentration in target tissues, also allowing the use of more potent agents which would be too toxic to administer in their unbound form. Two such ADCs have been approved for clinical use and many others are in development. Here we review the characteristics of each of the ADC components that have led to efficacious therapies and discuss some of the tubulin inhibitor-based ADCs in development for cancer therapy.

99mTc-labeled small-molecule inhibitors of prostate-specific membrane antigen: pharmacokinetics and biodistribution studies in healthy subjects and patients with metastatic prostate cancer

Prostate-specific membrane antigen (PSMA) is a well-established target for developing radiopharmaceuticals for imaging and therapy of prostate cancer (PCa). We have recently reported that novel (99m)Tc-labeled small-molecule PSMA inhibitors bind with high affinity to PSMA-positive tumor cells in vitro and localize in PCa xenografts. This study reports the first, to our knowledge, human data in men with metastatic PCa and in healthy male subjects.

METHODS

Under an exploratory investigational new drug, using a cross-over design, we compared the pharmacokinetics, biodistribution, and tumor uptake of (99m)Tc-MIP-1404 and (99m)Tc-MIP-1405 in 6 healthy men and 6 men with radiographic evidence of metastatic PCa. Whole-body images were obtained at 10 min and 1, 2, 4, and 24 h. SPECT was performed between 3 and 4 h after injection.

RESULTS

Both agents cleared the blood rapidly, with MIP-1404 demonstrating significantly lower urinary activity (7%) than MIP-1405 (26%). Both agents showed persistent uptake in the salivary, lacrimal, and parotid glands. Uptake in the liver and kidney was acceptable for imaging at 1-2 h. In men with PCa, both agents rapidly localized in bone and lymph node lesions as early as 1 h. SPECT demonstrated excellent lesion contrast. Good correlation was seen with bone scanning; however, more lesions were demonstrated with (99m)Tc-MIP-1404 and (99m)Tc-MIP-1405. The high-contrast images exhibited tumor-to-background ratios from 3:1 to 9:1 at 4 and 20 h.

CONCLUSION

Compared with the standard-of-care bone scanning, (99m)Tc-MIP-1404 and (99m)Tc-MIP-1405 identified most bone metastatic lesions and rapidly detected soft-tissue PCa lesions including subcentimeter lymph nodes. Because (99m)Tc-MIP-1404 has minimal activity in the bladder, further work is planned to correlate imaging findings with histopathology in patients with high-risk metastatic PCa.