Prediction of receptor properties and binding affinity of ligands to benzodiazepine/GABAA receptors using artificial neural networks.

To date the use of artificial neural networks (ANNs) in quantitative structure-activity relationship (QSAR) studies has been primarily concerned in comparing the predictive accuracy of the technique using known data sets where the data set parameters had been preselected and optimized for use with other statistical methods. Little effort has been directed at optimizing the input parameters for use with ANNs or exploring other potential strengths of ANNs. In this study, back-propagation ANNs and multilinear regression (MLR) were used to examine the QSAR between substituent constants and random noise at six positions on 57 1,4-benzodiazepin-2-ones (1,4-BZs) and their binding affinities (log IC50) for benzodiazepine GABAA receptor preparations. By using selective pruning and cross-validation techniques, it was found possible to use ANNs to indicate an optimum set of 10 input parameters from a choice of 48 which were then used to train back-propagation ANNs that best predicted the receptor binding affinity with a high correlation between known and predicted data sets. Using the optimum set of input parameters, three-layer ANNs performed no better than the two-layer ANNs which gave marginally better results than MLR. Using the trained ANNs to examine the individual parameters showed that increases in the lipophilicity and F polar value at position 7, F polar value at position 2', and dipole at position 1 on the molecule all enhanced receptor binding affinity of 1,4-BZ ligands. Increases in molar refractivity and resonance parameters at position 1, molar refractivity at positions 6' and 2', Hammet meta constant at position 3', and Hammet para constant at position 8 on the molecule all caused decreases in receptor binding affinity. By considering the optimal ANNs as pharmacophore models representing the internal physicochemical structure of the receptor site, it was found that they could be used to critically examine the properties of the receptor site.

Physiological modeling of disposition of potential tumor-imaging radiopharmaceuticals in tumor-bearing mice.

Radiopharmaceuticals have great potential in the early detection of human tumors. Three potential 99mTc-labeled platinum compounds based on cisplatin have been synthesized and tested in tumored mice. This report presents the analysis of the disposition data obtained after a single intravenous injection with an empirical, physiologically based pharmacokinetic model. The radioactivity of each radiopharmaceutical after administration was measured in blood, urine, and 15 tissues, including tumor. Parameters included in the model were tissue volumes (experimentally determined), tissue blood flows (determined from literature values), tissue:blood extraction ratios (determined by nonlinear least-squares regression with MULTI-FORTE), and clearance terms (also determined by nonlinear least-squares regression). Data were weighted by the reciprocal of the square of the observed values. Good fits to the experimental data were obtained. As expected, the compound producing the best tumor:blood profile (3) also had the highest tumor extraction ratio (6.2 versus 2.0 and 1.3 for 1 and 2, respectively). Total body clearance values for the radioactivity associated with the three compounds 1-3 were calculated to be 0.09, 0.04, and 0.016 mL/min, respectively. Analysis of data with such an empirical, physiologically based model may assist future development of suitable tumor-imaging agents.

Genotoxicity and cell cycle effects of platinum-based imaging agents in cisplatin-resistant human tumour cells.

A series of cis-dichloroplatinum(II) 2,3-diaminopropionamide complexes synthesised as potential imaging agents was tested for activity against a human ovarian tumour cell line (CI-80-13S) with high natural resistance to cisplatin and carboplatin as compared with other human cells. The most potent compound, the dimethyl ester of dichloro-[4-(methyleneiminodiacetic acid)phenyl (2',3'-diamino-propionamide)]platinum(II) (complex III), exhibited toxicity towards CI-80-13S cells similar to that observed in other cell lines, an effect that was not shown by the ligand alone or by cis-dichloroplatinum(II) 2,3-diaminopropionamide. However, complex III ester reproduced the genotoxic effects of cisplatin as judged by differential inactivation of two strains of adenovirus and by inhibition of cellular DNA and RNA synthesis; no major differences in these properties were observed between CI-80-13S and cisplatin-sensitive cells. Substantial inhibition of DNA and RNA synthesis was found within 2 h of treatment, much earlier than the effect of cisplatin. Complex III ester, which was 30- to 100-fold less potent than cisplatin, inhibited cell cycle progression in a similar way to equitoxic cisplatin, with cells accumulating in G2 at a dose of low toxicity and being arrested in all stages at higher levels. The latter in combination with colcemid caused extensive fragmentation of CI-80-13S cells. These results suggest that the mechanism of toxicity of such complexes involves factors, in addition to DNA damage, which rapidly inhibit nucleic acid synthesis and overcome natural resistance to cisplatin in the CI-80-13S cell line.

Biological evaluation of 99mTc labelled cis-platinum containing benzyliminodiacetic acids as potential tumour imaging agents.

The synthesis of three new potential tumour-imaging radiopharmaceuticals in which a cis-platin derivative is attached to benzyl iminodiacetic acid, a ligand capable of forming a stable complex with 99mTc, has previously been reported by us [Awaluddin et al. Appl. Radiat. Isot. 38, 671-674 (1987)]. We have now carried out extensive biodistribution studies on these compounds as well as on two fragments of their structures which do not contain platinum. The results suggest that the presence of platinum is not essential for the tumour-localizing properties of the radiopharmaceuticals.

Structure-distribution studies on some 99mTc-o-hydroxybenzyliminodiacetic acid complexes.

Biodistributions of a series of thirteen 99mTc-o-hydroxy-benzyliminodiacetic acid complexes were carried out in rats and their hepatobiliary and urinary outputs correlated with lipophilicity, molecular weight, influence of substituent and plasma protein binding. Hepatobiliary output was moderate for those ligands with large alkyl substituents [t-butyl (36%), and iso-octyl (42%)] but compared to HIDA compounds was relatively low, indicating that they would not be suitable for clinical use. Halogen substituents had only a small effect on increasing hepatobiliary output but a large effect on reducing the urinary clearance.

Synthesis and characterization of potential tumor scintigraphic agents.

Potential tumor imaging radiopharmaceutical agents have been prepared by attaching a cisplatin derivative to a ligand capable of forming a stable complex with 99mTc. Three new organometallic compounds, with iminodiacetic acid as the 99mTc chelating group and 2,3-diaminopropionamide as the platinum complexing group, have been prepared and characterized. Preliminary biodistribution studies in tumor bearing mice support the utility of this approach.

Zirconium molybdate gel as a generator for technetium-99m--II. High activity generators.

The characteristics of zirconium molybdate gels were described in Part I of this paper. Here it is shown that zirconium molybdate gel, prepared from neutron irradiated molybdenum oxide with 99Mo specific activities between 4.3 and 13.5 GBq g-1 is sufficiently stable for use as a matrix in preparation of high activity 99mTc generators. Elution efficiencies of generators containing from 2 to 6 g of gel ranged from 83 to 50% for 10 mL elutions. Generators containing 150 g of gel were eluted with efficiencies of 80-98%, with the activity peak in the first 50 mL. Radionuclidic impurities were considerably below the limits set by the British Pharmacopoeia. Although the pH of eluates was less than 4, subsequent treatment with a small zirconium oxide bed provided adjustment to within BP limits, and reduced 99Mo impurity levels tenfold to values near 10(-4)%. No significant differences were found between the biodistributions in rats of radiopharmaceuticals prepared with pertechnetate from gel generators and those from other generators.

Phenolic aminocarboxylate chelates of 99mTc as hepatobiliary agents.

A series of alkyl- and halogen-substituted derivatives of ethylenediamine di[o-hydroxyphenylacetic acid] (EDDHA) and N,N'-bis[2-hydroxybenzyl] ethylenediamine N,N'-diacetic acid (HBED) were complexed with 99mTc and their biodistribution was determined in rats. All complexes displayed substantial hepatobiliary excretion; of each series, 99mTc-Br-EDDHA and 99mTc-di-Cl-HBED had the maximum amount in the gastrointestinal tract. Scintigraphic studies of 99mTc-Cl-EDDHA in dogs revealed prompt imaging of the liver followed by imaging of the gall bladder as the complex was excreted into the bile.

Technetium-99m benzimidazolyl iminodiacetic acid hepatobiliary radiopharmaceuticals: structure biodistribution studies.

The biodistribution of fifteen structural variants of the hepatobiliary radiopharmaceutical, technetium-99m benzimidazolyl iminodiacetic acid (BIMIDA) were determined 1 h after i.v. injection into rats. The best compounds with respect to hepatobiliary excretion were those with halogen substituents in the benzene ring of the BIMIDA ligand. The cholescintigraphic properties of the BIMIDA compounds compared favourably with those of technetium-99m acetanilido iminodiacetic acid (HIDA) radiopharmaceuticals.