Molecular imaging of tumor hypoxia: Evolution of nitroimidazole radiopharmaceuticals and insights for future development.

Though growing evidence has been collected in support of the concept of dose escalation based on the molecular level images indicating hypoxic tumor sub-volumes that could be radio-resistant, validation of the concept is still a work in progress. Molecular imaging of tumor hypoxia using radiopharmaceuticals is expected to provide the required input to plan dose escalation through Image Guided Radiation Therapy (IGRT) to kill/control the radio-resistant hypoxic tumor cells. The success of the IGRT, therefore, is heavily dependent on the quality of images obtained using the radiopharmaceutical and the extent to which the image represents the true hypoxic status of the tumor in spite of the heterogeneous nature of tumor hypoxia. Available literature on radiopharmaceuticals for imaging hypoxia is highly skewed in favor of nitroimidazole as the pharmacophore given their ability to undergo oxygen dependent reduction in hypoxic cells. In this context, present review on nitroimidazole radiopharmaceuticals would be immensely helpful to the researchers to obtain a birds-eye view on what has been achieved so far and what can be tried differently to obtain a better hypoxia imaging agent. The review also covers various methods of radiolabeling that could be utilized for developing radiotracers for hypoxia targeting applications.

188 Re-N-DEDC Lipiodol for Treatment of Hepatocellular Carcinoma (HCC)-A Clinical and Prospective Study to Assess In-Vivo Distribution in Patients and Clinical Feasibility of Therapy.

Objective The incidence of inoperable hepatocellular carcinoma (HCC) with/without malignant portal vein thrombosis (PVT) is increasing in India for the last decade; thus, Bhabha Atomic Research Centre (BARC), Mumbai, India, developed diethydithiocarbamate (DEDC), a new transarterial radionuclide therapy (TART) agent. 188 Re-N-DEDC lipiodol is an emerging radiotherapeutic agent for inoperable HCC treatment due to its simple and onsite labeling procedure, cost-effectiveness, and least radiation-induced side effects. This study aimed to evaluate in-vivo biodistribution and clinical feasibility of 188 Re-N-DEDC lipiodol TART in HCC and optimization of labeling procedure to assess post-labeling stability and radiochemical yield of labeled lipiodol with 188 Re-N-DEDC complex. Materials and Methods DEDC kits were obtained as gift from BARC, Mumbai. Therapy was given to 31 HCC patients. Post-therapy planar and single-photon emission computed tomography/computed tomography (SPECT/CT) imaging were performed to see tumor uptake and biodistribution. Clinical feasibility and toxicity were decided by Common Terminology Criteria for Adverse Events version 5.0 (CTCAE v 5.0). Statistical Analysis Descriptive statistics was done for data using SPSS v22. Values was expressed as mean ± standard deviation or median with range. Results Post-therapy planar and SPECT/CT imaging showed radiotracer localization in hepatic lesions. Few patients showed lungs uptake due to hepato-pulmonary shunt (lung shunt < 10%). Maximum clearance was observed through urinary tract with very less elimination through hepatobiliary route due to slow rate of leaching of tracer. No patient showed myelosuppression or any other long-term toxicity over median follow-up of 6 months. Mean overall % radiochemical yield of 188 Re-N-DEDC lipiodol was 86.04 ± 2.35%. The complex 188 Re-N-DEDC was found to be stable at 37°C under sterile condition over a period of 1 hour without any significant change on the % radiochemical purity (90.83 ± 3.24%, 89.78 ± 3.67%, 89.22 ± 3.77% at 0, 0.5, 1 hours, respectively). Conclusion Human biodistribution showed very high retention of radiotracer in hepatic lesions with no long-term toxicity with this therapy. The kit preparation procedure is ideally suited for a busy hospital radio-pharmacy. By this procedure, 188 Re-N-DEDC lipiodol can be prepared in high radiochemical yield within a short time (∼45 minutes). Thus, 188 Re-N-DEDC lipiodol can be considered for TART in advanced and/or intermediate HCC.

Technetium-99m labeled core shell hyaluronate nanoparticles as tumor responsive, metastatic skeletal lesion targeted combinatorial theranostics.

Achieving target specific delivery of chemotherapeutics in metastatic skeletal lesions remains a major challenge. Towards this, a dual drug loaded, radiolabeled multi-trigger responsive nanoparticles having partially oxidized hyaluronate (HADA) conjugated to alendronate shell and palmitic acid core were developed. While the hydrophobic drug, celecoxib was encapsulated in the palmitic acid core, the hydrophilic drug, doxorubicin hydrochloride was linked to the shell via a pH responsive imine linkage. Hydroxyapatite binding studies showed affinity of alendronate conjugated HADA nanoparticles to bones. Enhanced cellular uptake of the nanoparticles was achieved via HADA-CD44 receptor binding. HADA nanoparticles demonstrated trigger responsive release of encapsulated drugs in the presence of hyaluronidase, pH and glucose, present in excess in the tumor microenvironment. Efficacy of the nanoparticles for combination chemotherapy was established by >10-fold reduction in IC50 of drug loaded particles with a combination index of 0.453, as compared to free drugs in MDA-MB-231 cells. The nanoparticles could be radiolabeled with the gamma emitting radioisotope technetium-99m (99mTc) through a simple, 'chelator free', procedure with excellent radiochemical purity (RCP) (>90 %) and in vitro stability. 99mTc-labeled drug loaded nanoparticles reported herein constitutes a promising theranostic agent to target metastatic bone lesions. STATEMENT OF HYPOTHESES: Technetium-99m labeled, alendronate conjugated, dual targeting, tumor responsive, hyaluronate nanoparticle for tumor specific drug release and enhanced therapeutic effect, with real-time in vivo monitoring.

Preparation of Rhenium-188-Lipiodol Using Freeze-Dried Kits for Transarterial Radioembolization: An Overview and Experience in a Hospital Radiopharmacy.

Background: Rhenium-188(188Re)-lipiodol is a clinically effective, economically viable radiopharmaceutical for Selective Internal Radiation Therapy of liver cancer. Present study evaluates the performance of three freeze-dried kits with respect to the radiochemistry, quality control, and overall "ease of preparation" aspects in a hospital radiopharmacy. Materials and Methods: Freeze-dried kits of acetylated 4-hexadecyl-4,7-diaza-1,10-decanedithiol (AHDD), super six sulfur (SSS), and diethyl dithiocarbamate (DEDC), obtained commercially or received as gift, were used for the preparation of 188Re-lipiodol using freshly eluted 188Re-sodium perrhenate from commercial Tungsten-188/188Re generator following recommended procedures. Results: The overall yield of 188Re-lipiodol prepared using AHDD Kit, SSS Kit, and DEDC Kit was 74.82% ± 3.3%, 87.55% ± 4.8%, and 76.38% ± 4.6%, respectively. Observed radiochemical purity (RCP) of 188Re-lipiodol prepared using these kits was 88.65% ± 2.8%, 92.92% ± 3.0%, and 91.38% ± 3.0%, respectively. Using a modified version of the DEDC Kits, overall yield of 87.17% ± 2.7% and RCP of 95.43% ± 2.3% could be achieved. Conclusions: While all three freeze-dried kits can be used for the preparation of 188Re-lipiodol in >70% overall yield, the modified version of DEDC Kits has some advantages in terms of preparation time and volume of Rhenium-188 activity that can be added to the kit vial. The latter feature of the DEDC Kit is particularly useful for patient dose preparation with 188Re activity of low radioactive concentration.

Clinical Management of Liver Cancer in India and Other Developing Nations: A Focus on Radiation Based Strategies.

Hepatocellular carcinoma (HCC) is a global killer with preponderance in Asian and African countries. It poses a challenge for successful management in less affluent or developing nations like India, with large populations and limited infrastructures. This review aims to assess the available options and future directions for management of HCC applicable to such countries. While summarizing current and emerging clinical strategies for detection, staging and therapy of the disease, it highlights radioisotope- and radioactivity-based strategies as part of an overall program. Using the widely accepted Barcelona Clinic Liver Cancer (BCLC) staging system as a base, it evaluates the applicability of different therapeutic approaches and their synergistic combination(s) in the context of a patient-specific dynamic results-based strategy. It distills the conclusions of multiple HCC management-focused consensus recommendations to provide a picture of clinical strategies, especially radiation-related approaches. Additionally, it discusses the logistical and economic feasibility of these approaches in the context of the limitations of the burdened public health infrastructure in India (and like nations) and highlights possible strategies both at the clinical level and in terms of an administrative health policy on HCC to provide the maximum possible benefit to the widest swathe of the affected population.

68Ga-labeled PET tracers for targeting tumor hypoxia: Role of bifunctional chelators on pharmacokinetics.

INTRODUCTION: By virtue of their oxygen dependant accumulation in hypoxic cells, radiolabeled nitroimidazole analogues have been widely used for detecting tumor hypoxia. Present study evaluates two 2-nitroimidazole (2-NIM) based 68Ga-labeled radiotracers, [68Ga]Ga-DOTAGA-2-NIM and [68Ga]Ga-NODAGA-2-NIM, for hypoxia targeting applications. METHODS: Bifunctional chelating agents suitable for radiolabeling with 68Ga, viz. 1,4,7,10-tetraazacyclododececane,1-(glutaric acid)-4,7,10-triacetic acid (DOTAGA) and 1,4,7-triazacyclododececane,1-(glutaric acid)-4,7-diacetic acid (NODAGA), were coupled to appropriately modified 2-nitroimidazole to obtain 2-NIM-DOTAGA and 2-NIM-NODAGA, respectively. These ligands were radiolabeled using [68Ga]GaCl3 obtained from a commercial 68Ge/68Ga-generator to obtain corresponding 68Ga-complexes. Both the radiotracers were tested for their hypoxia selectivity in CHO cells under hypoxic and normoxic conditions. Biodistribution studies in fibrosarcoma tumor bearing Swiss mice were carried out to evaluate the radiotracer in vivo. RESULTS: The 68Ga complexes of 2-NIM-DOTAGA and 2-NIM-NODAGA could be prepared in ~82% and ~90% yield, respectively. In vitro studies of the complexes in CHO cells showed significant accumulation of [68Ga]Ga-NODAGA-2-NIM complex under hypoxic conditions with hypoxic to normoxic ratio of 2.88 ± 0.36 at 180 min post incubation. The [68Ga]Ga-DOTAGA-2-NIM complex also showed hypoxia selectivity albeit to a lesser extent. Biodistribution studies of the complexes in Swiss mice bearing fibrosarcoma tumor showed significant tumor uptake by both radiolabeled complexes. [68Ga]Ga-NODAGA-2-NIM showed a more favorable pharmacokinetics with respect to [68Ga]Ga-DOTAGA-2-NIM. CONCLUSION: The nitroimidazole radiotracer with NODAGA chelator displayed more favorable pharmacokinetics and good hypoxia selectivity, making it a promising candidate for further investigation. The present study also provides an insight into the possible role of bifunctional chelator on overall pharmacokinetics of small molecule radiotracers.

Multifunctional Core-Shell Glyconanoparticles for Galectin-3-Targeted, Trigger-Responsive Combination Chemotherapy.

Galectin-3 (gal-3) plays a crucial role in various cellular events associated to tumor metastasis and progression. In this direction, gal-3 binding core-shell glyconanoparticles based on citrus pectin (CP) have been designed for targeted, trigger-responsive combination drug delivery. Depolymerization via periodate oxidation in heterogeneous medium yielded low-molecular weight dialdehyde oligomers (CPDA) of CP with a gal-3 binding property (Kd = 160.90 µM). CPDA-based core-shell nanoparticles prepared to enhance the gal-3 binding specificity via a multivalent ligand presentation have shown to reduce homotypic cellular aggregation, tumor cell binding with endothelial cells, and endothelial tube formation, the major steps involved in the progression of cancer. Immune-fluorescence and flow cytometric analysis confirmed significant reduction in gal-3 expression on MDA-MB 231 cancer cells upon incubation with nanoparticles. An on-demand tumor microenvironment-responsive release of drugs at low pH and high concentrations of glucose and glutathione prevailing in tumor milieu was achieved by introducing a cleavable Schiff's base, a boronate ester, and disulfide linkages within the shell of the nanoparticles. Nanoparticles with encapsulated sulindac in the core and doxorubicin (DOX) in the shell demonstrated target specificity and enhanced internalization with synergistic cytotoxic effects with a 30-fold reduction in IC50 in DOX-resistant, triple-negative MDA-MB 231 breast cancer cells. Nanoparticles were radiolabeled with 131I radioisotopes with ≥80% efficiency while retaining its gal-3 binding property. Biodistribution studies of radiolabeled placebo nanoparticles and drug-loaded CPDA nanoparticles demonstrated proof of concept of gal-3 targeting seen as preferential accumulation in the gal-3-expressing tissues of the gastric tract. The CPDA core-shell nanoparticles are thus promising platforms for gal-3 targeting and inhibition of gal-3-mediated processes involved in cancer progression with a potential of radiolabeling for in vivo monitoring or delivering therapeutic doses of radiation and on-demand triggered, target-specific drug release.

Synthesis and evaluation of 99mTc-analogues of [123/131I]mIBG prepared via [99mTc][Tc(CO)3(H2O)3]+ synthon for targeting norepinephrine transporter.

INTRODUCTION: meta-[123/131I]Iodobenzylguanidine (mIBG) is a clinical agent used for imaging neuroendocrine tumors, where uptake in tumor is via active transport mechanism through norepinephrine transporters (NET). Our group in past have evaluated a 99mTc-analogue of the above tracer, based on 99mTc-4 + 1 labeling approach, which exhibited significant affinity for NET but suffered from reduced specific uptake in comparison to reference standard no-carrier-added (n.c.a.) [125I]mIBG. The present work attempts to synthesize two new 99mTc-analogues of the radio-iodinated derivative following [99mTc]Tc(CO)31+ approach with an aim to improve the above specific uptake content. METHODS: Two different precursors, xylylenediamine and 1,3-bis(chloromethyl)benzene, were synthetically modified to yield meta-functionalized benzylguanidine derivatives bearing iminodiacetate (IDA) and aminoethylglycine (AEG) tridentate chelating moieties, respectively. These ligands were labeled with technetium-99m via [99mTc][Tc(CO)3(H2O)3]+ synthon to form desired radioactive complexes 9 and 10. The radiolabeling yields of the complexes obtained were >90% as confirmed by radio-HPLC. The HPLC purified complexes were used for in vitro and in vivo evaluation to understand the true biological efficacy. Structural characterization of the radiolabeled complexes was carried after synthesizing and characterizing their Re-analogues. RESULTS: Cell uptake studies with the radiolabeled complexes in SK-N-SH neuroblastoma cell lines revealed reduced uptake in the cells (<1% of incubated radioactivity/106 cells) in comparison to n.c.a. [125I]mIBG (~12%). However, limited specificity (~60%) was observed for the complexes as ascertained through desmethylimipramine (DMI) inhibition. Biodistribution studies in normal Wistar rats exhibited desired non-target clearance pharmacokinetics for the complexes but in vivo NET efficacy in myocardium for the neutral complex 10 could not be established. CONCLUSIONS: Tridentate [99mTc]Tc(CO)31+ chelation approach severely affects biological behavior of the present small bioactive molecule under study to a significant extent in comparison to monodentate ligation in 99mTc-4 + 1 strategy.

Clinical utility of 188Rhenium-hydroxyethylidene-1,1-diphosphonate as a bone pain palliative in multiple malignancies.

188Rhenium-hydroxyethylidene-1,1-diphosphonate (188Re-HEDP) is a clinically established radiopharmaceutical for bone pain palliation of patients with metastatic bone cancer. Herein, the effectiveness of 188Re-HEDP for the palliation of painful bone metastases was investigated in an uncontrolled initial trial in 48 patients with different types of advanced cancers. A group of 48 patients with painful bone metastases of lung, prostate, breast, renal, and bladder cancer was treated with 2.96-4.44 GBq of 188Re-HEDP. The overall response rate in this group of patients was 89.5%, and their mean visual analog scale score showed a reduction from 9.1 to 5.3 (P < 0.003) after 1 week posttherapy. The patients did not report serious adverse effects either during intravenous administration or within 24 h postadministration of 188Re-HEDP. Flare reaction was observed in 54.2% of patients between day 1 and day 3. There was no correlation between flare reaction and response to therapy (P < 0.05). Although bone marrow suppression was observed in patients receiving higher doses of 188Re-HEDP, it did not result in any significant clinical problems. The present study confirmed the clinical utility and cost-effectiveness of 188Re-HEDP for palliation of painful bone metastases from various types of cancer in developing countries.

Synthesis and biodistribution studies of 99m Tc labeled fatty acid derivatives prepared via "Click approach" for potential use in cardiac imaging.

123 I-Iodophenylpentadecanoic acid (IPPA) is a metabolic agent used in nuclear medicine for diagnosis of myocardial defects. Efforts are underway worldwide to develop a 99m Tc substitute of the above radiopharmaceutical for the aforementioned application. Herein, we report synthesis and biodistribution studies of 99m Tc labeled fatty acids (8, 11, and 15 carbons) obtained via "click chemistry" for its potential use in myocardial imaging. ω-Bromo fatty acids (8C/11C/15C) were synthetically modified at bromo terminal to introduce a heterocyclic triazole with glycine sidearm in a five step procedure. Modified fatty acids were subsequently radiolabeled with preformed [99m Tc(CO)3 ]+ synthon to yield the desired fatty acid complexes which were evaluated in Swiss mice. All the radiolabeled complexes were obtained with radiochemical purities >80%, as characterized by HPLC. Biodistribution studies of all three complexes in Swiss mice showed myocardial uptake of ~6-9% ID/g at 2 minutes post-injection, close to* I-IPPA (~9% ID/g). Complexes exhibited significant retention in the myocardium up to 30 minutes (~1% ID/g) but were lower to the standard agent (~7% ID/g). Similar uptake of activity in myocardium for the newly synthesized complexes in comparison to 125 I-IPPA along with favorable in vivo pharmacokinetics merits potential for the present "click" design of complexes for myocardial imaging.

Improved freeze-dried kit for the preparation of 188ReN-DEDC/lipiodol for the therapy of unresectable hepatocellular carcinoma.

Rhenium-188-N-(DEDC)2/lipiodol (abbreviated as 188ReN-DEDC, where DEDC = monoanionic diethyldithiocarbamate) is a clinically proven radiopharmaceutical for the therapy of unresectable hepatocellular carcinoma (HCC) through trans arterial radioembolization (TARE). A two-vial freeze-dried kit for the preparation of [188ReN(DEDC)2] complex using sodium perrhenate (Na188ReO4) obtained from a commercial Tungsten-188/Rhenium-188 generator had been reported earlier. This method required addition of stipulated volume of glacial acetic acid into vial 1 by the user for efficient preparation of [188ReN]2+ intermediate. An error in this step can result in low radiochemical yield of [188ReN]2+ intermediate as well as sub-optimal pH of the reaction mixture for the second step, leading to poor radiochemical purity of 188ReN-DEDC complex. In the present work, a solution to this problem was found by including an oxalate buffer of pH = 3 in vial 1, eliminating the need for the addition of glacial acetic acid by the user. This modification not only made the kits more user-friendly, it resulted in significant improvement in the kinetics of formation of [188ReN]2+ intermediate, wherein > 95% radiochemical purity could be achieved within 5 min incubation at ambient temperature. Moreover, the novel route for the preparation of [188ReN]2+ intermediate may be applied to any radiopharmaceutical based on 188ReN-core.

In Vitro Evaluation of 188Re-HEDP: A Mechanistic View of Bone Pain Palliations.

Skeletal metastasis is common in advanced stages of various cancers, particularly of the prostate and breast carcinoma. 188Re-HEDP (1-hydroxyethane 1, 1-diphosphonic acid) is a clinically established radiopharmaceutical for bone pain palliation of osseous metastasis, and it takes advantage of high bone affinity. The present work aims at elucidating the possible mechanisms of cell killing by 188Re-HEDP in osteosarcoma cells and biodistribution studies in mice.188Re-HEDP complex was prepared by using lyophilized HEDP kits prepared in-house. In vitro cellular uptake in mineralized bone matrix was found to be 13.41% ± 0.46% (at 2 hours), which was reduced to 2.44% ± 0.12% in the presence of excess amounts of unlabeled HEDP ligand. Uptake of 188Re-HEDP in bones of normal Swiss mice in vivo and mineralized bone in vitro indicated its affinity toward the bone matrix. The study also revealed that cellular toxicity and G2/M cell cycle arrest were dose dependent. At higher doses, G2/M cell cycle arrest was observed, which might be the major cause of cell death and a possible mechanism of bone pain relief.

A Freeze-Dried Kit for the Preparation of (188)Re-HEDP for Bone Pain Palliation: Preparation and Preliminary Clinical Evaluation.

(188)Re-HEDP is an established radiopharmaceutical used for pain palliation in patients with osseous metastasis. Considering commercial availability of (188)W/(188)Re generator, the accessibility to a lyophilized kit would make preparation of this radiopharmaceutical feasible at the hospital radiopharmacy having access to a generator. A protocol for the preparation of a single-vial lyophilized hydroxyethane 1,1-diphosphonic acid (HEDP) kit was developed and its consistency was checked by preparing six batches. Each sterile lyophilized kit prepared as per the protocol contained 9 mg of HEDP, 3 mg of gentisic acid, and 4 mg of SnCl2.2H2O. Randomly selected kits from all six batches were subjected to thorough quality control tests that were passed by all batches. (188)Re-HEDP could be prepared by addition of 1 mL of freshly eluted Na(188)ReO4 (up to 3700 MBq) containing 1 µmol of carrier ReO4(-) (perrhenate) and heating at 100°C for 15 minutes. (188)Re-HEDP with >95% radiochemical purity could be consistently prepared using the lyophilized kits. Sterile (188)Re-HEDP prepared using the lyophilized kit was evaluated in patients with osseous metastasis. Post-therapy images of the patient were compared with (99m)Tc-MDP bone scan and found to be satisfactory. The bone-to-background as well as tumor-to-normal bone uptake ratio was found to be significant. All patients who received therapy reported significant pain relief within a week to 10 days post-administration of (188)Re-HEDP.

Recognition-mediated cucurbit[7]uril-heptamolybdate hybrid material: a facile supramolecular strategy for (99m)Tc separation.

We report the construction of a novel non-covalently held cucurbit[7]uril-heptamolybdate hybrid material for the first time, and demonstrate its application as a generator bed for the facile and efficient separation of the (99m)Tc radiotracer, which is in demand for several theranostic applications.

Modulation of in vivo distribution through chelator: Synthesis and evaluation of a 2-nitroimidazole-dipicolylamine-(99m)Tc(CO)3 complex for detecting tumor hypoxia.

Previous studies have clearly demonstrated strong correlation between in vivo distribution and blood clearance of radiopharmaceuticals for the detection of hypoxia. Present study describes an attempt to improve the in vivo distribution of a previously reported 2-nitroimidazole-(99m)Tc(CO)3 complex by tuning its blood clearance pattern through structural modification of the ligand. Herein, a 2-nitroimidazole-dipicolylamine ligand (2-nitroimidazole-DPA) was synthesized in a two-step procedure and radiolabeled with (99m)Tc(CO)3 core. Subsequently, the complex was evaluated in Swiss mice bearing fibrosarcoma tumor. As intended by its design, 2-nitroimidazole-DPA-(99m)Tc(CO)3 complex was more lipophilic than previously reported 2-nitroimidazole-DETA-(99m)Tc(CO)3 complex (DETA-diethylenetriamine) and showed slower blood clearance. Consequently it showed higher tumor uptake than 2-nitroimidazole-DETA-(99m)Tc(CO)3 complex. Significantly, despite structural modifications, other parameters such as the tumor to blood ratio and tumor to muscle ratio of the 2-nitroimidazole-DPA-(99m)Tc(CO)3 complex remained comparable to that of 2-nitroimidazole-DETA-(99m)Tc(CO)3 complex. Present study demonstrates the feasibility of structural modifications for improving in vivo tumor uptake of hypoxia detecting radiopharmaceuticals. This might encourage researchers to improve suboptimal properties of a potential radiopharmaceuticals rather than ignoring it altogether.

A (99m)Tc-labeled misonidazole analogue: step toward a (99m)Tc-alternative to [18F]fluromisonidazole for detecting tumor hypoxia.

The PET radiopharmaceutical [(18)F]Fluromisonidazole ([(18)F]FMISO) is presently the agent of choice for the clinical imaging of tumor hypoxia. Considering the logistic advantages of (99m)Tc and wider availability of SPECT machines, a (99m)Tc-radiopharmaceutical for this purpose constitutes an attractive choice. In the work presented here, a misonidazole analogue was radiolabeled with (99m)Tc(CO)3 core and the complex was evaluated in Swiss mice bearing fibrosarcoma tumor. The results obtained are compared with the biodistribution of [(18)F]FMISO carried out in the same tumor-bearing animal model. Misonidazole-(99m)Tc(CO)3 complex showed significant uptake and retention in tumor. Notably, the rate of clearance of misonidazole complex from the tumor was slower than that of [(18)F]FMISO. The maximum tumor/muscle ratio obtained with misonidazole-(99m)Tc(CO)3 complex was significantly higher than that of [(18)F]FMISO. The study constitutes a positive step toward the development of a (99m)Tc-analogue of [(18)F]FMISO.

A study on nitroimidazole-99mTc(CO)3 complexes as hypoxia marker: some observations towards possible improvement in in vivo efficacy.

INTRODUCTION: Hypoxia plays a negative role in the clinical management of cancer. Detection of hypoxic status of a cancer is important for selecting patients for hypoxia directed therapy. Though [(18)F]fluoromisonidazole ([(18)F]FMISO), a PET radiopharmaceutical, is presently being used in the clinic for the detection of hypoxia, considering the logistical advantages of (99m)Tc and wider availability of SPECT scanners, a radiopharmaceutical based on this isotope may find wider applicability. METHODS: Nine nitroimidazole (2-, 4- and 5-nitroimidazole) ligands were synthesized and radiolabeled using [(99m)Tc(CO)3(H2O)3](+) precursor to obtain a group of complexes possessing different single electron reduction potential (SERP), overall charge and lipophilicity, the three attributes which decide the efficacy of the complex to detect hypoxic cells in vivo. The nitroimidazole-(99m)Tc(CO)3 complexes as well as [(18)F]FMISO were evaluated in fibrosarcoma tumor bearing mice. RESULTS: The (99m)Tc(CO)3 complexes of nitroimidazole iminodiacetic acid (IDA) showed better tumor uptake and retention than nitroimidazole diethylenetriamine (DETA) and nitroimidazole aminoethylglycine (AEG) complexes. Tumor uptake observed with [(18)F]FMISO was higher than any of the nitroimidazole-IDA- (99m)Tc(CO)3 complexes. However, [(18)F]FMISO clearance from tumor was found to be faster compared to 2-nitroimidazole-IDA-(99m)Tc(CO)3 complex. Observed tumor uptake and retention of the radiotracers evaluated could be correlated to its blood clearance pattern and SERP. CONCLUSIONS: Results of the present study indicated that uptake of the radiotracer in tumor is closely associated with its rate of clearance from blood. The study also indicated that along with SERP, clearance of radiotracer from blood (net effect of charge and lipophilicity) is a critical factor which decides the in vivo efficacy of the hypoxia detecting radiopharmaceutical.

Preparation and evaluation of a 99mTcN-PNP complex of sanazole analogue for detecting tumor hypoxia.

A sanazole derivative, having a favorable single electron reduction potential (SERP) value compared to that of misonidazole, was synthesized and radiolabeled with [(99m)TcN(PNP)] precursor to evaluate its potential as a hypoxia imaging agent. The complex, which was lipophilic, could be prepared in good yields and challenging studies with cysteine showed stability of the complex against trans-chelation. However, despite being lipophilic as well as possessing favorable SERP value, biodistribution studies of this complex in fibrosarcoma tumor bearing Swiss mice showed low uptake in tumor. This observation is possibly attributed to fast clearance of the complex from blood, whereby the complex spends insufficient time in tumor to get reduced and trapped. Though uptake in tumor was low, slow clearance of activity from tumor suggests reduction and trapping of the complex in hypoxic cells. The present (99m)Tc-complex demonstrated acceptable values of tumor to blood (TBR) and tumor to muscle (TMR) ratios. However, low uptake in tumor which may not be indicative of the actual hypoxic status of the tumor, limit the utility of the complex to detect tumor hypoxia.

On the structural modification of 2-nitroimidazole-(99m)Tc(CO)(3) complex, a hypoxia marker, for improving in vivo pharmacokinetics.

INTRODUCTION: A 2-nitroimidazole-(99m)Tc(CO)(3) complex reported earlier showed promise with respect to its uptake and retention in hypoxic tumor. However, significant uptake and slow clearance from liver imposed severe limitations towards advocating its possible practical utility. In an attempt to improving its liver clearance, an ether linkage, which is known to help in liver clearance, was introduced in the molecule. METHODS: The 2-nitroimidazole iminodiacetic acid (IDA) derivative containing an ether linkage was synthesized in a five step procedure from 2-nitroimidazole. This derivative was radiolabeled using [(99m)Tc(CO)(3)(H(2)O)(3)](+) precursor complex. The corresponding Re(CO)(3) analogue was also synthesized in the macroscopic level for structural characterization. The (99m)Tc(CO)(3) complex was evaluated in an animal model bearing fibrosarcoma tumor. RESULTS: The in vivo evaluation of the complex indicated that, as envisaged, introduction of the ether linkage has improved clearance from the liver. The complex also showed higher retention in tumor compared to the 2-nitroimidazole-IDA-(99m)Tc(CO)(3) complex reported earlier. Though the tumor to muscle ratio improved with time, the tumor to blood ratio did not show any significant improvement. Despite improved liver clearance, there was significant liver activity present even at 3h p.i. attributable to gradual accumulation of activity cleared from muscle and blood. CONCLUSIONS: Though the introduction of ether linkage improved liver clearance of the modified 2-nitroimidazole complex, it was found that a single ether linkage was not sufficient to achieve the desirable level of clearance. Probably, a linker with multiple ether groups, such as a di- or tri-ethylene glycol spacer, may be a possible solution to this issue.