Radiometals as Payloads for Radioimmunotherapy for Lymphoma Lymphoma

A first-in-class dual-chelator theranostic agent designed for use with imaging-therapy radiometal pairs of different elements

A covalent adduct of DFOB and DOTA separated by a l-lysine residue (DFOB-l-Lys-N 6-DOTA) exhibited remarkable regioselective metal binding, with {1H}-13C NMR spectral shifts supporting Zr(iv) coordinating to the DFOB unit, and Lu(iii) coordinating to the DOTA unit. This first-in-class, dual-chelator theranostic design could enable the use of imaging-therapy radiometal pairs of different elements, such as 89Zr for positron emission tomography (PET) imaging and 177Lu for low-energy β--particle radiation therapy. DFOB-l-Lys-N 6-DOTA was elaborated with an amine-terminated polyethylene glycol extender unit (PEG4) to give DFOB-N 2-(PEG4)-l-Lys-N 6-DOTA (compound D2) to enable installation of a phenyl-isothiocyanate group (Ph-NCS) for subsequent monoclonal antibody (mAb) conjugation (mAb = HuJ591). D2-mAb was radiolabeled with 89Zr or 177Lu to produce [89Zr]Zr-D2-mAb or [177Lu]Lu-D2-mAb, respectively, and in vivo PET/CT imaging and in vivo/ex vivo biodistribution properties measured with the matched controls [89Zr]Zr-DFOB-mAb or [177Lu]Lu-DOTA-mAb in a murine LNCaP prostate tumour xenograft model. The 89Zr-immuno-PET imaging function of [89Zr]Zr-D2-mAb and [89Zr]Zr-DFOB-mAb showed no significant difference in tumour accumulation at 48 or 120 h post injection. [89Zr]Zr-D2-mAb and [177Lu]Lu-D2-mAb showed similar ex vivo biodistribution properties at 120 h post-injection. Tumour uptake of [177Lu]Lu-D2-mAb shown by SPECT/CT imaging at 48 h and 120 h post-injection supported the therapeutic function of D2, which was corroborated by similar therapeutic efficacy between [177Lu]Lu-D2-mAb and [177Lu]Lu-DOTA-mAb, both showing a sustained reduction in tumour volume (>80% over 65 d) compared to vehicle. The work identifies D2 as a trifunctional chelator that could expand capabilities in mixed-element radiometal theranostics to improve dosimetry and the clinical outcomes of molecularly targeted radiation.

Molecular targeted α-particle therapy for oncologic applications

OBJECTIVE

A significant challenge facing traditional cancer therapies is their propensity to significantly harm normal tissue. The recent clinical success of targeting therapies by attaching them to antibodies that are specific to tumor-restricted biomarkers marks a new era of cancer treatments.

CONCLUSION

In this article, we highlight the recent developments in α-particle therapy that have enabled investigators to exploit this highly potent form of therapy by targeting tumor-restricted molecular biomarkers.

Methods for radiolabelling of monoclonal antibodies

The use of radionuclide labels allows to study the pharmacokinetics of monoclonal antibodies, to control the specificity of their targeting and to monitor the response to an antibody treatment with high accuracy. Selection of label depends on the processing of an antibody after binding to an antigen, and on the character of information to be derived from the study (distribution of antibody in the extracellular space, target occupancy or determination of sites of metabolism). This chapter provides protocols for labelling of antibodies with iodine-125 (suitable also for other radioisotopes of iodine) and with indium-111. Since radiolabelling might damage and reduce the immunoreactive fraction and/or affinity of an antibody, the methods for assessment of these characteristics of an antibody are provided for control.

Antibody delivery of drugs and radionuclides: factors influencing clinical pharmacology

The therapeutic rationale of antibody conjugates is the selective delivery of a cytotoxin to tumor cells via binding and internalization of the monoclonal antibodies to a specific cell-surface antigen, thereby enhancing the therapeutic index of the cytotoxin. The key structural and functional components of an antibody conjugate are the antibody, the linker and the cytotoxin (chemical or radionuclide) with each component being critical for the successful development of the conjugate. Considerable efforts have been made in understanding the pharmacokinetics, pharmacodynamics, tissue distribution, metabolism and pharmacologic effects of these complex macromolecular entities. The purpose of this article is to discuss the properties and various structural components of antibody conjugates that influence their clinical pharmacology.

Quantitative serial imaging of an 124I anti-CEA monoclonal antibody in tumor-bearing mice

OBJECTIVE

The 4.2-day half-life of (124)I favors its use for positron emission tomography (PET) of monoclonal antibodies (mAbs). However, high positron energy and beta(+) -associated cascade gamma rays pose image resolution and background noise problems for (124)I. This study evaluated quantitative PET of an (124)I mAb in tumor-bearing mice.

METHODS

An R4 microPETtrade mark (Siemens/CTIMI, Knoxville, TN) was used with standard energy and coincidence timing windows (350-750 keV and 6 ns, respectively), delayed random coincidence subtraction, iterative image reconstruction, and no attenuation or scatter correction. Image resolution, contrast, and response linearity were compared for (124)I and (18)F, using phantoms. Nude mice bearing human colon tumors (LS-174T) were injected intravenously with a chimeric (124)I anti-CEA mAb (cT84.66) and imaged serially 1 hour to 7 days postinjection. Venous blood was sampled to validate image-derived blood curves. Mice were sacrificed after the final scan, and the biodistribution of (124)I was measured by direct tissue assay. Images were converted to units of kBq/g for each tissue of interest by comparing the final scans with the direct assays.

RESULTS

Measured resolution (FWHM) 0-16 mm from the scanner axis was 2.3-2.7 mm for (124)I versus 1.9-2.0 mm for (18)F. Due to true coincidence events between annihilation photons and cascade gamma rays, background was greater for (124)I than (18)F, but the signal-to-background ratio was still more than 20, and (124)I image intensities varied linearly with activity concentration. Tissue-based calibration worked well (i.e., PET blood curves agreed with direct measurements within 12% at all time points), while calibration, based on a cylindrical phantom approximating the mouse body, yielded tumor quantitation that was 46%-66% low, compared with direct assay.

CONCLUSIONS

Images of quantitative accuracy sufficient for biodistribution measurements can be obtained from tumor-bearing mice by using (124)I anti-CEA mAbs with standard microPET acquisition and processing techniques, provided the calibration is based on the direct assay of excised tissue samples.

Development of TNKase-specific cleavable peptide-linked radioimmunoconjugates for radioimmunotherapy

Radioimmunotherapy (RIT) is a method for selectively delivering radionuclides to cancer cells while reducing the radiation dose to normal tissues. However, because of slow clearance of MAbs, normal tissues also received radiotoxicity. One of the promising strategies is linking on-demand cleavable (ODC) peptides between radiometal chelates and the tumor targeting agents. We have tested this proof-of-concept by using ODC peptides that are designed to be cleaved only by TNKase and are resistant to cleavage by enzymes present in the plasma and the tumor. TNKase-specific peptide linkers using l- and d-amino acids were screened by OBOC combinatorial peptide libraries. One of the best peptides was linked to radiometal chelate and ChL6-MAb to prepare radioimmunoconjugate (RIC). Optimization and characterization of the linker conjugation to MAb show (a) 1-2 peptides linked to each MAb; (b) immunoreactivity >80%; (c) specific activity of the RIC 0.7-1 microCi/microg; (d) RIC stable over 7 days in human plasma; and (e) radiometal-chelated ODC peptide cleaved from the RIC in plasma by TNKase at clinical dose levels of 10 microg/ml. The percent release of radiochelate from RIC was 50% at 24h and 85% over 7 2h in vitro. This novel ODC-linked RIC could be a potential molecule for RIT.

Positron emission tomography (PET) imaging of neuroblastoma and melanoma with 64Cu-SarAr immunoconjugates

The advancement of positron emission tomography (PET) depends on the development of new radiotracers that will complement (18)F-FDG. Copper-64 ((64)Cu) is a promising PET radionuclide, particularly for antibody-targeted imaging, but the high in vivo lability of conventional chelates has limited its clinical application. The objective of this work was to evaluate the novel chelating agent SarAr (1-N-(4-aminobenzyl)-3, 6,10,13,16,19-hexaazabicyclo[6.6.6] eicosane-1,8-diamine) for use in developing a new class of tumor-specific (64)Cu radiopharmaceuticals for imaging neuroblastoma and melanoma. The anti-GD2 monoclonal antibody (mAb) 14.G2a, and its chimeric derivative, ch14.18, target disialogangliosides that are overexpressed on neuroblastoma and melanoma. Both mAbs were conjugated to SarAr using carbodiimide coupling. Radiolabeling with (64)Cu resulted in >95% of the (64)Cu being chelated by the immunoconjugate. Specific activities of at least 10 microCi/microg (1 Ci = 37 GBq) were routinely achieved, and no additional purification was required after (64)Cu labeling. Solid-phase radioimmunoassays and intact cell-binding assays confirmed retention of bioactivity. Biodistribution studies in athymic nude mice bearing s.c. neuroblastoma (IMR-6, NMB-7) and melanoma (M21) xenografts showed that 15-20% of the injected dose per gram accumulated in the tumor at 24 hours after injection, and only 5-10% of the injected dose accumulated in the liver, a lower value than typically seen with other chelators. Uptake by a GD2-negative tumor xenograft was significantly lower (<5% injected dose per gram). MicroPET imaging confirmed significant uptake of the tracer in GD-2-positive tumors, with minimal uptake in GD-2-negative tumors and nontarget tissues such as liver. The (64)Cu-SarAr-mAb system described here is potentially applicable to (64)Cu-PET imaging with a broad range of antibody or peptide-based imaging agents.

Development of radioimmunotherapeutic and diagnostic antibodies: an inside-out view

Only a handful of radiolabeled antibodies (Abs) have gained US Food and Drug Administration (FDA) approval for use in clinical oncology, including four immunodiagnostic agents and two targeted radioimmunotherapeutic agents. Despite the advent of nonimmunogenic Abs and the availability of a diverse library of radionuclides, progress beyond early Phase II radioimmunotherapy (RIT) studies in solid tumors has been marginal. Furthermore, [18F]fluorodeoxyglucose continues to dominate the molecular imaging domain, underscored by a decade-long absence of any newly approved Ab-based imaging agent (none since 1996). Why has the development of clinically successful Abs for RIT been limited to lymphoma? What obstacles must be overcome to allow the FDA approval of immuno-positron emission tomography (immuno-PET) imaging agents? How can we address the unique challenges that have thus far prevented the introduction of Ab-based imaging agents and therapeutics for solid tumors? Many poor decisions have been made regarding radiolabeled Abs, but useful insight can be gained from these mistakes. The following review addresses the physical, chemical, biological, clinical, regulatory and financial limitations that impede the progress of this increasingly important class of drugs.

Radiotherapy of CD45-Expressing Daudi Tumors in Nude Mice with Yttrium-90-Labeled, PEGylated Anti-CD45 Antibody

Studies were performed to determine the suitability of using the polyethylene glycol (PEG)-labeled AHN-12 anti-CD45 monoclonal antibody to deliver the high-energy beta-particle-emitting isotope 90Y to a CD45+ B-cell Daudi lymphoma grown as flank tumors in athymic nude mice. The PEGylated radiolabeled antibody displayed a significantly better antitumor effect in the mouse tumor flank model (p<0.03) and significantly better blood pharmacokinetics in normal rats (p<0.05) than the non-PEGylated radiolabeled antibody. Studies of two different sizes of PEG showed that rats given 43 kDa of PEGylated AHN-12, but not 5 kDa of PEGylated AHN-12, had significantly higher radiolabeled antibody blood levels and, therefore, improved pharmacokinetics, as compared to rodents given non-PEGylated radiolabeled AHN-12 (p<0.05). Surviving mice revealed no signs of kidney, liver, or gastrointestinal damage by histology study. Notably, in vitro studies indicated that PEGylation did not have a major effect on labeling efficiency and the binding of labeled antibody. These findings indicate that PEGylation of radiolabeled anti-CD45 antibody may be a useful and desirable means of extending blood half-life and enhancing efficacy. Also, the final outcome may be impacted by the size of the PEG molecule used for the modification of the blood half-life.

Treatment of Non-Hodgkin’s Lymphoma (NHL) With Radiolabeled Antibodies (mAbs)

Most patients with non-Hodgkin's lymphoma (NHL) achieve remission but, despite newer drugs, the natural history of this disease has not improved during the last 20 years. Less than one half of patients with aggressive NHL are cured, and few of those with low-grade NHL are curable. Furthermore, NHL becomes progressively more chemoresistant while remaining responsive to external beam radiation therapy. Radioimmunotherapy (RIT) is a logical strategy for the treatment of NHL because this disease is multifocal and radiosensitive. Because of their remarkable effectiveness for RIT, 2 anti-CD20 monoclonal antibodies (mAbs), one labeled with (111)In for imaging or (90)Y for therapy and a second labeled with (131)I for imaging and therapy, have been approved for use in patients with NHL. These drugs have proven remarkably effective and safe. Evidence for the importance of the radionuclide is manifested by the data in the randomized pivotal phase III trial of (90)Y-ibritumomab that revealed response rates were several times greater in the (90)Y-ibritumomab arm than in the rituximab arm. A second drug for RIT, (131)I-tositumomab, was compared in a pivotal trial with the efficacy of the last chemotherapy received by each patient. Once again, response rates were much higher for RIT. Both (90)Y-ibritumomab and (131)I-tositumomab require preinfusion of several hundred milligrams of unlabeled anti-CD20 mAb to obtain "favorable" biodistribution, that is, targeting of NHL. Response rates for other mAbs and radionuclides in NHL also have been high but these drugs have not reached the approval stage. These drugs can be used safely by physicians who have suitable training and judgment. Unlike chemotherapy, RIT is not associated with mucositis, hair loss, or persistent nausea or vomiting. Although hematologic toxicity is dose limiting, hospitalization for febrile neutropenia is uncommon. Randomized trials of RIT in different formulations have not been conducted, but there is evidence to suggest that the mAb, antigen, radionuclide, chelator, linker, and dosing strategy may make a difference in the outcome.

Regional Radiochemotherapy Using In Situ Hydrogel

PURPOSE

To evaluate the feasibility of regional radiochemotherapy of mammary tumors using in situ hydrogel loaded with cisplatin (CDDP) and rhenium-188 ((188)Re).

METHODS

Sodium alginate (SA) and calcium chloride were used to create a hydrogel for delivery of CDDP and (188)Re. In vitro studies were performed to evaluate cytotoxic effects of (188)Re-hydrogel and sustained-release ability of the CDDP-hydrogel. Tumor-bearing rats were injected with (188)Re-hydrogel (0.5-1 mCi/rat), (188)Re-perrhenate (0.5-1 mCi/rat, intratumoral, I.T.), CDDP-hydrogel (3 mg/kg), and (188)Re-hydrogel loaded with CDDP (3 mg/kg body weight, 0.5-1 mCi/rat), respectively, and groups receiving (188)Re were imaged at 24 and 48 h postinjection. Tumor volume, body weight, imaging, and kidney function were assessed as required for each group.

RESULTS

Successful formation of the hydrogel was demonstrated by cytotoxic effects of (188)Re-hydrogel and slow release of CDDP-hydrogel in vitro. Tumor volume measurements showed significant delay in tumor growth in treated vs. control groups with minimal variation in normal kidney function for the CDDP-hydrogel group. Scintigraphic images indicated localization of (188)Re-hydrogel in the tumor site up to 48 h postinjection.

CONCLUSIONS

Our data demonstrate the feasibility of using hydrogel for delivery of chemotherapeutics and radiation locally. This technique may have applications involving other contrast modalities as well as treatment in cases where tumors are inoperable.