Anupam Mathur

On the isolation and evaluation of a novel unsubstituted 5-nitroimidazole derivative as an agent to target tumor hypoxia.

The presence and extent of hypoxic regions in cancerous tissue bears a negative influence on the effectiveness of radiation therapy and chemotherapy of the cancer hence estimation of hypoxia is an important problem. Several (99m)Tc-labeled nitroimidazole-based non-invasive agents have been tried for this purpose but none had optimal characteristics and the search continues. Herein we report, for the first time to the best of our knowledge, the isolation, (99m)Tc(CO)(3) labeling and evaluation of an unsubstituted 5-nitroimidazole derivative obtained as a side product during the synthesis of 4-nitroimidazole derivative. The (99m)Tc(CO)(3)-labeled complex of 5-nitroimidazole derivative could be prepared in excellent yield under mild conditions. Its evaluation in fibrosarcoma tumor bearing Swiss mice showed uptake and slow clearance of injected activity from tumor. The tumor-to-muscle ratio was found to be very high but tumor-to-blood ratio greater than 1 could not be obtained throughout the limited time point study. The study revealed that complex under investigation has features similar to other 2-nitroimidazole complexes so far as the retention of injected activity in tumor is concerned.

On the structural modification of 2-nitroimidazole-99mTc(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.

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.

Synthesis and bio-evaluation of a new fatty acid derivative for myocardial imaging.

Development of a (99m)Tc-fatty acid analogue is of interest, as (99m)Tc is logistically advantageous over the cyclotron-produced (11)C and (123)I. Synthesis of a 16 carbon fatty acid derivative and its radiolabeling with the novel [(99m)TcN(PNP)](2+) core is described here. Hexadecanedioic acid was conjugated to cysteine in an overall yield of 55%. This ligand could be labeled with (99m)Tc via the [(99m)TcN(PNP)](2+) core, in 80% yield, as a mixture of two isomers (syn and anti). The major isomer isolated by HPLC was used for bioevaluation studies in swiss mice and compared with radioiodinated iodophenyl pentadecanoic acid (IPPA), an established agent for myocardial metabolic imaging. (99m)Tc-labeled complex cleared faster from the non-target organs, namely, liver, lungs, and blood compared to that of [(125)I]-IPPA. However, the complex exhibited lower uptake and faster washout from the myocardium as compared to [(125)I]-IPPA.

Development of 68Ga‐labeled fatty acids for their potential use in cardiac metabolic imaging

While [(11)C]palmitate continues to be a promising tracer for cardiovascular Positron Emission Tomography (PET) imaging, unfavourable logistics due to the short half-life of (11)C (20 min) and cumbersome labeling methodologies are the major impediments that limit its widespread use. In order to circumvent such limitations, an attempt has been made to explore the potential of (68)Ga-labeled fatty acid analogs for metabolic imaging owing to the availability of (68)Ga through a (68)Ge/(68)Ga generator on an on-demand basis. In this study, two fatty acid conjugates were synthesized by conjugation of p-SCN-benzyl NOTA with the ω-amino group of 11-amino undecanoic acid and 12-amino dodecanoic acid, respectively, under alkaline conditions. Both derivatives were radiolabeled in high yields with (68)Ga obtained from an in-house (68)Ge/(68)Ga generator. Biodistribution studies in Swiss mice showed reasonable myocardial uptake at 2 min for both derivatives (7.4 ± 2.8% ID/g for 11-carbon fatty acid-NOTA conjugate and 6.4 ± 2.1% ID/g for 12-carbon fatty acid-NOTA conjugate), which cleared rapidly over 30 min. However, significant activity was found in blood for both tracers, with heart/blood ratios observed to be below 0.5 at all time points, diminishing the potential of the synthesized complexes for cardiac imaging.

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.

A 99mTc-Labeled Misonidazole Analogue: Step Toward a 99mTc-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.

Efficient production of therapeutic doses of [131I]-metaiodobenzylguanidine for clinical use

[(131)I]-metaiodobenzylguanidine (mIBG) is a known radiopharmaceutical used for the treatment of neuroendocrine tumors. The development of therapeutic [(131)I]-mIBG doses at production level is highly challenging due to rapid product degradation and high radiation exposures to the production plant personnel. In the present work, a working module for the production of 10 doses (100 mCi each) in a single operation was developed following copper (I) assisted isotope exchange. The labeled product complies with the pharmaceutical specifications suitable for in-vivo patient use.

Syntheses and biological evaluation of 99mTc-HYNIC-fatty acid complexes for myocardial imaging

The aim of the present study is to identify a 99mTc-labeled fatty acid tracer which could be a possible substitute of the widely used 123I-labeled fatty acids in studying myocardial metabolism and in detection of myocardial abnormalities in high-risk patients. The relevance of the study stems from the fact that in terms of wider applicability, a 99mTc-tracer is expected to be more advantageous compared to that of a 123I-based one. Two fatty acid (FA)-hydrazinopyridine-3-carboxylic acid (HYNIC) conjugates (11C-FA-HYNIC and 12C-FA-HYNIC) were synthesized and radiolabeled with 99mTc using two different co-ligands system viz. tricine/ethylenediamine diacetic acid (EDDA), and tricine/trisodium triphenylphosphine-3,3′,3′′-trisulfonate (TPPTS), to yield four radiolabeled complexes. While all four 99mTc-HYNIC-complexes showed uptake in the myocardium, 12C-FA-HYNIC-99mTc-EDDA complex showed higher uptake and retention in myocardium compared to other complexes. In general, uptake of the 99mTc-complexes in non-target organs was lower than that of 125I-iodophenyl pentadecanoic acid (IPPA). The 12C-FA-HYNIC-99mTc-EDDA complex, additionally exhibited lower liver accumulation compared to that of 125I-IPPA. Though these features were favorable for cardiac imaging, the heart-to-blood ratio of the complexes were low (<1). Nevertheless, a dynamic SPECT image of 12C-FA-HYNIC-99mTc-EDDA complex in Swiss mouse showed delineation of its myocardium from proximal non-target organs. The results merit further screening of synthetically modified 99mTc-HYNIC fatty acids for myocardial imaging.

68 Ga labeled fatty acids for cardiac metabolic imaging: Influence of different bifunctional chelators

Development of 68Ga labeled fatty acids is of immense interest due to the availability of 68Ga through a generator and its superiority over SPECT based tracers in carrying out dynamic imaging on a PET scanner. Our present work explores the influence of different chelators on the cardiac uptake and pharmacokinetics of the 68Ga-labeled fatty acids. Two new 68Ga labeled fatty acids were synthesized by conjugation of 11-aminoundecanoic acid with the bifunctional chelators (BFCs) viz. p-SCN-Bn-DTPA (S-2-(4-isothiocyanatobenzyl)-diethylenetriaminepentaacetic acid) and p-SCN-Bn-NODAGA (S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1-glutaric acid-4,7-acetic acid) and their comparison was carried out with the previously reported 68Ga-NOTA-undecanoic acid. Both the conjugates were radiolabeled with 68Ga in high yields and purities (>95%). Their formation was established by preparation and characterization of their inactive analogs with natGa at macroscopic levels. Biodistribution studies of the complexes in Swiss mice showed lower initial myocardial uptake for 68Ga-NODAGA-undecanoic acid (3.8±0.6%ID/g) and 68Ga-DTPA-undecanoic acid (1.3±0.5%ID/g) complexes in comparison to previously reported 68Ga-NOTA-undecanoic acid complex (7.4±2.8%ID/g) at 2min p.i. However, significant retention of the tracer in the myocardium was observed in the case of 68Ga-NODAGA-undecanoic complex, which led to improved heart/non-target ratios of the complex over time in comparison to the other 68Ga complexes. Similarly, the DTPA complex exhibited increased washout from the liver in comparison to other 68Ga derivatives. The β oxidation mechanism in myocytes was investigated by isolating the myocardial extract post intravenous injection of the respective 68Ga complexes and analyzing them by radio-HPLC, which showed metabolic transformation of the parent fatty acid complex peak in all the three complexes. This study has provided an insight into the design characteristics of 68Ga labeled fatty acids to achieve the desired myocardial imaging characteristics.