Comparative dosimetry of copper-64 and yttrium-90-labeled somatostatin analogs in a tumor-bearing rat model

Feasibility of Multiple Examinations Using (68)Ga-Labelled Collagelin Analogues: Organ Distribution in Rat for Extrapolation to Human Organ and Whole-Body Radiation Dosimetry

Objectives: Fibrosis is involved in many chronic diseases. It affects the functionality of vital organs, such as liver, lung, heart and kidney. Two novel imaging agents for positron emission tomography (PET) imaging of fibrosis have previously pre-clinically demonstrated promising target binding and organ distribution characteristics. However, the relevant disease monitoring in the clinical setup would require multiple repetitive examinations per year. Thus, it is of paramount importance to investigate the absorbed doses and total effective doses and thus, the potential maximum number of examinations per year. Methods: Two cyclic peptide (c[CPGRVMHGLHLGDDEGPC]) analogues coupled via an ethylene glycol linker (EG₂) to either 2-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazonan-1-yl)acetic acid (NO2A-Col) or 4-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazacyclononan-1-yl)-5-(tert-butoxy)-5-oxopentanoic acid (NODAGA-Col) were labelled with ⁶⁸Ga. The resulting agents, [⁶⁸Ga]Ga-NO2A-Col and [⁶⁸Ga]Ga-NODAGA-Col, were administered in the tail vein of male and female Sprague–Dawley rats (N = 24). An ex vivo organ distribution study was performed at the 5-, 10-, 20-, 40-, 60- and 120-min time points. The resulting data were extrapolated for the estimation of human organ and total body absorbed and total effective doses using Organ Level Internal Dose Assessment Code software (OLINDA/EXM 1.1) assuming a similar organ distribution pattern between the species. Time-integrated radioactivity in each organ was calculated by trapezoidal integration followed by a single-exponential fit to the data points extrapolated to infinity. The resulting values were used for the residence time calculation. Results: Ex vivo organ distribution data revealed fast blood clearance and washout from most of the organs. Although the highest organ absorbed dose was found for kidneys (0.1 mGy/MBq), this organ was not the dose-limiting one and would allow for the administration of over 1460 MBq per year for both [⁶⁸Ga]Ga-NO2A-Col and [⁶⁸Ga]Ga-NODAGA-Col. The total effective dose was the limiting parameter with 0.0155/0.0156 (female/male) mSv/MBq and 0.0164/0.0158 (female/male) mSv/MBq, respectively, for [⁶⁸Ga]Ga-NO2A-Col and [⁶⁸Ga]Ga-NODAGA-Col. This corresponded to the total amount of radioactivity that could be administered per year of 643 and 621 MBq before reaching the annual limit of 10 mSv. Thus, up to six examinations would be possible. The residence time and organ absorbed doses in liver and spleen were higher for [⁶⁸Ga]Ga-NODAGA-Col as compared to [⁶⁸Ga]Ga-NO2A-Col. Conclusion: The limiting parameter for the administered dose was the total effective dose that would allow for at least six examinations per year that might be sufficient for adequate disease monitoring in longitudinal studies and a routine clinical setup.

Underscoring the influence of inorganic chemistry on nuclear imaging with radiometals

Over the past several decades, radionuclides have matured from largely esoteric and experimental technologies to indispensible components of medical diagnostics. Driving this transition, in part, have been mutually necessary advances in biomedical engineering, nuclear medicine, and cancer biology. Somewhat unsung has been the seminal role of inorganic chemistry in fostering the development of new radiotracers. In this regard, the purpose of this Forum Article is to more visibly highlight the significant contributions of inorganic chemistry to nuclear imaging by detailing the development of five metal-based imaging agents: (64)Cu-ATSM, (68)Ga-DOTATOC, (89)Zr-transferrin, (99m)Tc-sestamibi, and (99m)Tc-colloids. In a concluding section, several unmet needs both in and out of the laboratory will be discussed to stimulate conversation between inorganic chemists and the imaging community.

Zebularine significantly sensitises MEC1 cells to external irradiation and radiopharmaceutical therapy when administered sequentially in vitro

Zebularine is a cytidine analogue incorporated into DNA during replication, inhibiting DNA methyltransferase 1 (DNMT1), resulting in demethylation and changes in gene expression. Such modification may improve radiosensitivity in resistant lymphoma cells. The hypothesis of this study was that zebularine and radiation would synergistically inhibit cell growth and viability. Human MEC1 malignant B cells were incubated with 0-200 µM zebularine for 48 h. Media containing zebularine was removed, and the cells were irradiated with 0-2 Gy of either external beam irradiation or (177) Lu-DOTA-TATE, a radiolabelled somatostatin analogue. Concentration and viability were measured over 48-72 h. The proportion of apoptotic cells was identified using an active Caspase 3/7 assay. Zebularine inhibited growth of cells in a dose-dependent manner during exposure. No residual growth inhibition occurred following removal of the drug. Zebularine and external irradiation inhibited cell proliferation in a dose-dependent, synergistic interaction, but the effect on viability was additive. Treatment with zebularine and (177) Lu-DOTA-TATE resulted in less inhibition of proliferation (P = 0.0135), but a synergistic decrease in viability. Apoptotic fraction was much higher in cells irradiated with (177) Lu-DOTA-TATE than external irradiation. External irradiation induces growth arrest rather than apoptosis. Apoptosis is the primary effect of radiopharmaceutical therapy on tumour cells. Treatment with the methylation inhibitor, zebularine, appears to synergistically augment these natural effects in vitro, which could be exploited clinically.

Radiometal-labeled somatostatin analogs for applications in cancer imaging and therapy

The use of radiolabeled peptides for the diagnosis and therapy of cancer has increased greatly over the last few decades. Skillfully crafted peptide systems, which have high affinity for receptors that are overexpressed in human tumors, offer the potential to improve the characterization, grading, and eventual therapy of human cancer. Robust peptide systems can be labeled with radioactive atoms for imaging purposes using single-photon emission computed tomography and positron emission tomography technologies, or can be labeled with therapeutic nuclides for the efficient killing of tumor cells. This method-based review discusses one such class of receptor-targeted peptides and their radiolabeling with radioactive metals. The somatostatin receptor is upregulated in many types of cancer, and when labeled with a radiometal atom via a bifunctional chelate, can be employed as an agent for the imaging and radiotherapy of cancer. This review will discuss the methods used in the synthesis of the somatostatin peptides, conjugation with bifunctional chelators, and radiolabeling with metal radionuclides. Methods will also be presented for the in vitro and in vivo evaluation of the compounds produced.

Preparation and biological evaluation of copper-64-labeled tyr3-octreotate using a cross-bridged macrocyclic chelator

PURPOSE

Somatostatin receptors (SSTr) are expressed on many neuroendocrine tumors, and several radiotracers have been developed for imaging these types of tumors. For this reason, peptide analogues of somatostatin have been well characterized. Copper-64 (t(1/2) = 12.7 hours), a positron emitter suitable for positron emission tomography (PET) imaging, was shown recently to have improved in vivo clearance properties when chelated by the cross-bridged tetraazamacrocycle 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo(6.6.2)hexadecane (CB-TE2A) compared with 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA).

EXPERIMENTAL DESIGN

CB-TE2A and TETA were conjugated to the somatostatin analogue tyrosine-3-octreotate (Y3-TATE) for evaluation of CB-TE2A as a bifunctional chelator of 64Cu. The in vitro affinity of each compound for SSTr was determined using a homologous competitive binding assay. In vivo characteristics of both radiolabeled compounds were examined in biodistribution and microPET studies of AR42J tumor-bearing rats.

RESULTS

Cu-CB-TE2A-Y3-TATE (Kd = 1.7 nmol/L) and Cu-TETA-Y3-TATE (Kd = 0.7 nmol/L) showed similar affinities for AR42J derived SSTr. In biodistribution studies, nonspecific uptake in blood and liver was lower for 64Cu-CB-TE2A-Y3-TATE. Differences increased with time such that, at 4 hours, blood uptake was 4.3-fold higher and liver uptake was 2.4-fold higher for 64Cu-TETA-Y3-TATE than for 64Cu-CB-TE2A-Y3-TATE. In addition, 4.4-times greater tumor uptake was detected with 64Cu-CB-TE2A-Y3-TATE than with 64Cu-TETA-Y3-TATE at 4 hours postinjection. MicroPET imaging yielded similar results.

CONCLUSIONS

CB-TE2A appears to be a superior in vivo bifunctional chelator of 64Cu for use in molecular imaging by PET or targeted radiotherapy due to both improved nontarget organ clearance and higher target organ uptake of 64Cu-CB-TE2A-Y3-TATE compared with 64Cu-TETA-Y3-TATE.

An assessment of the effects of shell cross-linked nanoparticle size, core composition, and surface PEGylation on in vivo biodistribution

Amphiphilic core-shell nanoparticles have drawn considerable interest in biomedical applications. The precise control over their physicochemical parameters and the ability to attach various ligands within specific domains suggest shell cross-linked (SCK) nanoparticles may be used as multi-/polyvalent scaffolds for drug delivery. In this study, the biodistribution of four SCKs, differing in size, core composition, and surface PEGylation, was evaluated. To facilitate in-vivo tracking of the SCKs, the positron-emitting radionuclide copper-64 was used. By using biodistribution and microPET imaging approaches, we found that small diameter (18 nm) SCKs possessing a polystyrene core showed the most favorable biological behavior in terms of prolonged blood retention and low liver accumulation. The data demonstrated that both core composition, which influenced the SCK flexibility and shape adaptability, and hydrodynamic diameter of the nanoparticle play important roles in the respective biodistributions. Surface modification with poly(ethylene glycol) (PEG) had no noticeable effects on SCK behavior.

Dosimetry of 60/61/62/64Cu-ATSM: a hypoxia imaging agent for PET

PURPOSE

Cu-diacetyl-bis(N4-methylthiosemicarbazone (Cu-ATSM) is an effective marker for the delineation of hypoxic tissue. Dosimetry calculations by the established Medical Internal Radionuclide Dose (MIRD) approach were performed with both animal and patient data.

METHODS

Human absorbed dose estimates extrapolated from rat data were based on the biodistribution of 61Cu-ATSM in adult rats. Eighteen tissues were harvested and time-activity curves generated. The measured residence times and the MIRD S-values for 60Cu-ATSM were used to estimate human absorbed doses. The biodistribution of the tracer was directly measured in five patients injected with approximately 480 MBq of 60Cu-ATSM and imaged by positron emission tomography (PET) with a whole-body protocol. The combined data from all patients were used to derive organ residence times, and organ doses were calculated by MIRD methodology for 60Cu-ATSM, 61Cu-ATSM, 62Cu-ATSM, and 64Cu-ATSM.

RESULTS

Human absorbed dose estimates extrapolated from rat biodistribution data indicated that the kidneys appeared to be the dose-limiting organ (0.083 mGy/MBq) with a whole-body dose of 0.009 mGy/MBq. Based on the human PET imaging data, the liver appeared as the dose-limiting organ, with an average radiation dose of 0.064 mGy/MBq. The whole-body dose was 0.009 mGy/MBq and the effective dose was 0.011 mSv/MBq.

CONCLUSION

These relatively small absorbed doses to normal organs allow for the safe injection of 500-800 MBq of 60Cu-ATSM, which is sufficient for PET imaging in clinical trials.

Spectral Properties of Pro-multimodal Imaging Agents Derived from a NIR Dye and a Metal Chelator

Monomolecular multimodal imaging agents (MOMIAs) are able to provide complementary diagnostic information of a target diseased tissue. We developed a convenient solid-phase approach to construct two pro-MOMIAs (before incorporating radiometal) derived from 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and cypate, a near-infrared (NIR) fluorescent dye analogous to indocyanine green (ICG). The possible interaction between d orbitals of transition metal DOTA complexes or free metals and the p orbitals of cypate chromophore could quench the fluorescence of pro-MOMIAs. However, we did not observe significant changes in the spectral properties of cypate upon conjugation with DOTA and subsequent chelation with metals. The fluorescence intensity of the chelated and nonmetal-chelated PRO-MOMIAs remained fairly the same in dilute 20% aqueous dimethylsulfoxide (DMSO) solution (1 x 10(-6) M). Significant reduction in the fluorescence intensity of pro-MOMIAs occurred in the presence of a large excess of metal ions (>1 molar ratio for indium and 20-fold for a copper relative to pro-MOMIA). This study suggests the feasibility of using MOMIAs for combined optical and radioisotope imaging.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.

In vivo evaluation of copper-64-labeled monooxo-tetraazamacrocyclic ligands

Copper-64 (T(1/2)=12.7 h; beta(+): 0.653 MeV, 17.4%; beta(-): 0.578 MeV, 39%) has applications in positron emission tomography (PET) imaging and radiotherapy, and is conveniently produced on a biomedical cyclotron. Tetraazamacrocyclic ligands are the most widely used bifunctional chelators (BFCs) for attaching copper radionuclides to antibodies and peptides due to their relatively high kinetic stability. In this paper, we evaluated three monooxo-tetraazamacrocyclic ligands with different ring sizes and oxo group positions. H1 [1,4,7,10-tetraazacyclotridecan-11-one], H2 [1,4,8,11-tetraazacyclotetradecan-5-one] and H3 [1,4,7,10-tetraazacyclotridecan-2-one] were radiolabeled with (64)Cu in high radiochemical yields under mild conditions. The three (64)Cu-labeled complexes are all +1 charged, as determined by their electrophoretic mobility. While they demonstrated >95% stability in rat serum out to 24 h, both biodistribution and microPET imaging studies revealed high uptake and long retention of the compounds in major clearance organs (e.g., blood, liver and kidney), which suggests that (64)Cu dissociated from the complexes in vivo. Of the three complexes, (64)Cu-2(+), which has a cyclam backbone (1,4,8,11-tetraazacyclotetradecane), exhibited the lowest nontarget organ accumulation. The data from these studies may invalidate the candidacy of the monooxo-tetraazamacrocyclics as BFCs for copper radiopharmaceuticals. However, the data presented here suggest that neutral or negatively charged Cu(II) complexes of tetraazamacrocyclic ligands with a cyclam backbone (tetradecane) are optimal for copper radiopharmaceutical applications.

Low-Dose-Rate Irradiation by 131I Versus High-Dose-Rate External-Beam Irradiation in the Rat Pancreatic Tumor Cell Line CA20948

AIM

The rat pancreatic CA20948 tumor cell line is widely used in receptor-targeted preclinical studies because many different peptide receptors are expressed on the cell membrane. The response of the tumor cells to peptide radionuclide therapy, however, is dependent on the cell line's radiosensitivity. Therefore, we measured the radiosensitivity of the CA20948 tumor cells by using clonogenic survival assays after high-energy external-beam radiotherapy (XRT) in vitro. It can, however, be expected that results of high-dose-rate XRT are not representative for those after low-dose-rate radionuclide therapy (RT), such as peptide-receptor radionuclide therapy. Therefore, we compared clonogenic survival in vitro in CA20948 tumor cells after increasing doses of XRT or RT, the latter using (131)I.

METHODS

Survival of CA20948 cells was investigated using a clonogenic survival assay after RT by incubation with increasing amounts of (131)I, leading to doses of 1-10 Gy after 12 days of incubation (maximum dose rate, 0.92 mGy/min), or with doses of 1-10 Gy using an X-ray machine (dose rate, 0.66 Gy/min). Colonies were scored after a 12-day-incubation period. Also, the doubling time of this cell line was calculated.

RESULTS

We observed a dose-dependent reduction in tumor-cell survival, which, at low doses, was similar for XRT and RT. For high-dose-rate XRT, the quadratic over linear component ratio (alpha/beta) for CA20948 was 8.3 Gy, whereas that ratio for low-dose-rate RT was calculated to be 86.5 Gy. The calculated doubling time of CA20948 cells was 22 hours.

CONCLUSIONS

Despite the huge differences in dose rate, RT tumor cell-killing effects were approximately as effective as those of XRT at doses of 1 and 2 Gy, the latter being the common daily dose given in fractionated external-beam therapies. At higher doses, RT was less effective than XRT.

Somatostatin analogs and radiopeptides in cancer therapy

Since the discovery of somatostatin (sst) in 1973, numerous chemical and biological studies have been carried out to develop sst analogs with enhanced resistance to proteases and prolonged activity. Three highly potent sst analogs-octreotide, lanreotide, and vapreotide-are now available in the clinic, and demonstrate efficacy in the treatment of tumors of the pituitary and the gastroenteropancreatic tract. The most striking effect is the control of hormone hypersecretion associated with these tumors. Available data on growth suppression in patients indicate a limited antiproliferative action, tumor shrinkage is observed in 10-20% patients, and tumor stabilization in about half of the patients for duration of 8-16 months. Eventually, however, all patients escape from sst analog therapy with regard to both hormone hypersecretion and tumor growth, the only exception being observed in acromegalic patients who do not experience tachyphylaxis even after more than 10 years of daily octreotide injection. The mechanism underlying the escape phenomenon is not yet clarified. Regarding the molecular mechanisms involved in sst antineoplastic activity, both indirect and direct effects via specific somatostatin receptors (SSTRs) expressed in the target cells have be described. Direct action may result from blockade of mitogenic growth signal or induction of apoptosis following interaction with SSTRs. Indirect effects may be the result of reduced or inhibited secretion of growth-promoting hormones and growth factors that stimulate the growth of various types of cancer; also, inhibition of angiogenesis or influence on the immune system are important factors. Five SSTR subtypes have been identified so far, which are variably expressed in a variety of tumors such as gastroenteropancreatic (GEP) tumors, pituitary tumors, and carcinoid tumors. Although all five SSTR subtypes are linked to adenylate cyclase, they are now known to affect multiple other cellular signaling systems and hence they differentially participate in the regulation of the various cellular processes. The finding of several laboratories that SSTR-expressing tumors frequently contain two or more SSTR subtypes, and the recent discovery that SSTR subtypes might form homo/heterodimers to create a novel receptor with different functional characteristics, expand the array of selective SSTR activation pathways and subsequent intracellular signaling cascades. This may lead to improved clinical protocols that take into account possible synergistic interactions between the SSTR subtypes present on the same cancer cell. Radiolabeled sst analogs, such as [(111)In]-[diethylenetriamine pentaacetic acid (DTPA)-D-Phe(1)]-octreotide (OcreoScan), have proved to be very useful for tumor scintigraphy and internal radiotherapy of SSTR overexpressing tumors. The recent introduction of the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) considerably improved the stability of the radioconjugates, making possible the incorporation of a variety of radionuclides, such as (90)Y for receptor-mediated radionuclide therapy or (68)Ga for positron emission tomography (PET). Another promising area is the development of sst conjugates incorporating cytotoxic anticancer drugs.

Ga-66 labeled somatostatin analogue DOTA-DPhe1-Tyr3-octreotide as a potential agent for positron emission tomography imaging and receptor mediated internal radiotherapy of somatostatin receptor positive tumors

Radionuclide labeled somatostatin analogues selectively target somatostatin receptor (SSTR)-expressing tumors as a basis for diagnosis and treatment of these tumors. Recently, a DOTA-functionalized somatostatin analogue, DOTATOC (DOTA-DPhe1-Tyr3-octreotide) has been developed. This compound has been shown to be superior to the other somatostatin analogues as indicated by its uniquely high tumor-to-non-target tissue ratio. DOTATOC can be labeled with a variety of radiometals including gallium radioisotopes. Gallium-66 is a positron emitting radionuclide (T(1/2) =9.5 hr; beta+=56%), that can be produced in carrier free form by a low-beam energy cyclotron. In this study we investigated SSTR targeting characteristics of 66Ga-DOTATOC in AR42J rat pancreas tumor implanted nude mice as a potential agent for diagnosis and receptor-mediated internal radiotherapy of SSTR-expressing tumors. We compared our results with 67Ga- and 68Ga- labeled DOTATOC. The radiolabeling procedure gave labeling yield ranged from 85-95% and radiochemical and chemical purity was > 95%. In-vitro competitive binding curves and in-vivo competitive displacement studies with an excess of unlabeled peptide indicates that there is specific binding of the radioligand to SSTR. Animal biodistribution data and serial microPET images demonstrated rapid tumor uptake and rapid clearance from the blood and all tissues except kidney. Maximum % ID/g values for tumor were 10.0 +/- 0.7, 13.2 +/- 2.1 and 9.8 +/- 1.5 for 66Ga-, 67Ga-, and 68Ga-DOTATOC, respectively. Calculated tumor, kidney and bone marrow doses for 66Ga-DOTATOC based on biodistribution data were 178, 109 and 1.2 cGy/MBq, respectively. We conclude that 66Ga labeled DOTATOC can be used for PET diagnosis and quantitative imaging-based dosimetry of SSTR positive tumors. 66Ga-DOTATOC may also be used in higher doses for ablation of these tumors. However, kidney is the critical organ for toxicity (tumor/kidney ratio = 1.64), and high kidney uptake must be eliminated before devising a therapy protocol.

Toxicity and dosimetry of (177)Lu-DOTA-Y3-octreotate in a rat model

Radiolabeled somatostatin analogs have demonstrated effectiveness for targeted radiotherapy of somatostatin receptor-positive tumors in both tumor-bearing rodent models and humans. A radionuclide of interest for cancer therapy is reactor-produced (177)Lu (t(1/2) = 6.64 d; beta(-) [100%]). The high therapeutic efficacy of the somatostatin analog (177)Lu-DOTA-Tyr(3)-octreotate (DOTA-Y3-TATE, where DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) was previously demonstrated in a tumor-bearing rat model (Erion et al., J. Nucl. Med. 1999;40:223P; de Jong et al., Int. J. Cancer, 2001; 92:628-633). In the current study, the toxicity and dosimetry of (177)Lu-DOTA-Y3-TATE were determined in both normal and tumor-bearing rats. Doses of (177)Lu-DOTA-Y3-TATE ranging from 0 to 123 mCi/kg were administered to rats and complete blood counts (CBCs) and blood chemistries were analyzed out to 6 weeks. No overt signs of toxicity were observed with (177)Lu-DOTA-Y3-TATE (i.e., lethargy, weight loss, scruffy coat or diarrhea) at any of the dose levels. Blood chemistries and CBCs were normal except for the white blood cell counts, which showed a dose-dependent decrease. The maximum tolerated dose was not reached at 123 mCi/kg. The biodistribution of (177)Lu-DOTA-Y3-TATE was determined in CA20948 rat pancreatic tumor-bearing rats, and the data were used to estimate human absorbed doses to normal tissues. The dose-limiting organ was determined to be the pancreas, followed by the adrenal glands. The absorbed dose to the rat CA20948 tumor was estimated to be 336 rad/mCi (91 mGy/MBq). These data demonstrate that (177)Lu-DOTA-Y3-TATE is an effective targeted radiotherapy agent at levels that show minimal toxicity in this rat model.