Steven D. Wyrick

Comparative neurotoxicity of oxaliplatin, ormaplatin, and their biotransformation products utilizing a rat dorsal root ganglia in vitro explant culture model.

Purpose: Neurotoxicity is one of the major toxicities of platinum-based anticancer drugs, especially oxaliplatin and ormaplatin. It has been postulated that biotransformation products are likely to be responsible for the toxicity of platinum drugs. In our preceding pharmacokinetic study, both oxaliplatin and ormaplatin were observed to produce the same types of major plasma biotransformation products. However, while the plasma concentration of ormaplatin was much lower than that of oxaliplatin at an equimolar dose, one of their common biotransformation products, Pt(dach)Cl2, was present at 29-fold higher concentrations in the plasma following the i.v. injection of ormaplatin than of oxaliplatin. Because ormaplatin has severe neurotoxicity and Pt(dach)Cl2 is very cytotoxic, we have postulated that Pt(dach)Cl2 is likely to be responsible for the differences in neurotoxicity between ormaplatin and oxaliplatin. In order to test this hypothesis, we compared the neurotoxicity of oxaliplatin, ormaplatin, and their biotransformation products. Since the dorsal root ganglia (DRGs) have been suggested to be the likely targtet for platinum drugs and in vitro DRG explant cultures have been suggested to be a valid model for studying cisplatin-associated neurotoxicity, our comparative neurotoxicity study was conducted with DRG explant cultures in vitro. Methods: Based on the previous studies of cisplatin neurotoxicity, we established our in vitro DRG explant culture utilizing DRGs dissected from E-19 embryonic rats. Rat DRGs were incubated for 30 min with different platinum compounds to mimic in vivo exposure conditions; this was by followed by a 48-h incubation in culture medium at 37 °C. At the end of the incubation, the neurites were fixed and stained with toluidine blue, and neurite outgrowth was quantitated by phase-contrast microscopy. The inhibition of neurite outgrowth by platinum compounds was used as an indicator of in vitro neurotoxicity. Since an in vivo study has indicated that the order of neurotoxicity is ormaplatin > cisplatin ≥ oxaliplatin > carboplatin as measured by morphometric changes to rat DRGs, we initially validated our DRG explant culture model by comparing the in vitro neurotoxicity of ormaplatin, cisplatin, oxaliplatin, and carboplatin. After observing the same neurotoxicity rank between this study and a previous in vivo study, we further compared the neurotoxicity of oxaliplatin, ormaplatin, and their biotransformation products including Pt(dach)Cl2, Pt(dach)(H2O)Cl, Pt(dach)(H2O)2, Pt(dach)(Met), and Pt(dach)(GSH) utilizing the DRG explant culture model. Results: Our study indicated that Pt(dach)Cl2 and its hydrolysis products were more potent at inhibiting neurite outgrowth than the parent drugs oxaliplatin and ormaplatin. In contrast, no detectable inhibition of neurite outgrowth was observed for DRGs dosed with Pt(dach)(Met) and Pt(dach)(GSH). Conclusion: This study suggests that biotransformation products such as Pt(dach)Cl2 and its hydrolysis products are more neurotoxic than the parent drugs oxaliplatin and ormaplatin. The different neurotoxicity profiles of oxaliplatin and ormaplatin are more likely due to the different plasma concentrations of their common biotransformation product Pt(dach)Cl2 than to differences in their intrinsic neurotoxicity.

High-performance liquid chromatographic separation of the biotransformation products of oxaliplatin

A novel single reversed-phase HPLC system was developed for separating oxaliplatin and its biotransformation products formed in rat plasma. The major stable biotransformation products of oxaliplatin formed in rat plasma were identified as Pt(dach)(Cys)2, Pt(dach)(Met) and free dach. The minor biotransformation products Pt(dach)Cl2, Pt(dach)(GSH) and Pt(dach)(GSH)2 could also be resolved from other Pt-dach complexes. Among these biotransformation products, the identification of Pt(dach)(Met) was further confirmed by LC-ESI-MS, and the identification of Pt(dach)(Cys)2, Pt(dach)(GSH), Pt(dach)(GSH)2 and free dach was confirmed by atomic absorption and double isotope labeling. This HPLC technique should prove useful for separating and identifying the biotransformation products of Pt-dach drugs such as oxaliplatin, ormaplatin and Pt(dach)(mal) in biological fluids. This will allow a more complete characterization of the pharmacokinetics and biotransformations of these Pt-dach drugs, which should in turn lead to a better understanding of the mechanisms leading to their toxicity and efficacy.

Hypolipidemic activity of in vitro inhibitors of hepatic and intestinal sn-glycerol-3-phosphate acyltransferase and phosphatidate phosphohydrolase

A number of agents including a series of 1,3-bis (substituted phenoxy)-2-propanones were screened in vitro for their ability to inhibit hepatic and intestinal microsomal sn-glycerol-3-phosphate acyltransferase and phosphatidate phosphohydrolase. Effective inhibitors reduced in vivo hepatic and intestinal glycerolipid production and with one exception also lowered serum triglyceride levels, suggesting that agents which inhibit potential rate-limiting steps of glycerolipid biosynthesis may be effective hypolipidemic agents. Two compounds, 1-methyl-4-piperidyl bis (p-chlorophenoxy) acetate (Sah 42-348) and 1,3-bis (p-methylphenoxy)-2-propanone were the best inhibitors of glycerolipid biosynthesis and lipid-lowering agents. The lipid-altering effects of both drugs were compared to chlorophenoxyisobutyrate during high fructose intake in rats. Each agent reduced fructose induced glycerolipid biosynthesis and serum triglyceride levels to similar degrees.

Displacement of the bidentate malonate ligand from (d,l-trans-1,2-diaminocyclohexane)malonatoplatinum(II)by physiologically important compounds in vitro

Previous studies of platinum(II) compounds with bidentate leaving ligands have emphasized the contrast between the stability of the bidentate leaving ligand in vitro (T1/2 greater than 11 days in water) and the apparent reactivity of these bidentate platinum compounds in vivo. However, none of these studies actually measured the stability of these compounds in tissue culture medium (or in any other reaction mixture resembling in vivo conditions). The experiments described in this paper were designed to measure the stability and fate of (d,l-trans-1,2-diaminocyclohexane)malonatoplatinum(II) [Pt(mal)(trans-dach)] in RPMI-1640 tissue culture medium. The T1/2 for displacement of the malonate ligand in this medium was 9.5 hr at 37 degrees. Of the inorganic anions present in the medium, chloride accounted for the greatest displacement of the malonate ligand. However, at the concentrations with which it is found in tissue culture medium and in blood, bicarbonate was nearly as effective as chloride at displacing the malonate ligand. This observation is of particular significance because the bicarbonatoplatinum complex is unstable and the bicarbonate displacement reaction appears to represent a major non-enzymatic pathway for the formation of the biologically active aquated platinum complexes. At the concentrations with which they occur inside the cell, phosphates may play a similar role. Of the amino acids present in the medium, glutathione and the sulfur-containing amino acids were 50- to 400-fold more effective at displacing the malonate ligand than the other amino acids in RPMI-1640 medium. In the case of methionine, the reaction with Pt(mal)(trans-dach) was shown to be a direct displacement (SN2) reaction at physiological methionine concentrations. When Pt(mal)(trans-dach) was incubated at 37 degrees for 24 hr in RPMI-1640 medium, the major transformation products formed were (d,l-trans-1,2-diaminocyclohexane)methionineplatinum(II) (38%), other amino acid-platinum complexes (19%), and (d,l,-trans-1,2-diaminocyclohexane)dichloroplatinum(II) (14%). Eleven percent of the Pt(mal)(trans-dach) remained intact. Mass spectrometry and 1H-NMR indicated that the (d,l-trans-1,2-diaminocyclohexane)methionineplatinum(II) complexes that formed in RPMI-1640 medium consisted of approximately 60% of the bidentate mono-methionine complex coordinated to platinum at the sulfur and alpha-amino positions and 40% of the bis-methionine complex, presumably coordinated at the sulfurs.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetics and biotransformations of oxaliplatin in comparison with ormaplatin following a single bolus intravenous injection in rats.

Purpose: Traditionally ultrafilterable Pt has been used to estimate the body exposure to platinum drugs. However, previous studies have shown that ultrafilterable Pt consists of both cytotoxic and inert biotransformation products of platinum drugs. Therefore, it has been proposed that pharmacokinetic parameters of the parent drug and its cytotoxic biotransformation products are more likely to be correlated with the drug toxicity and efficacy than those of ultrafilterable Pt. Oxaliplatin and ormaplatin are likely to form very similar biotransformation products in vivo based on previous studies. However, ormaplatin causes severe and irreversible neurotoxicity while oxaliplatin causes moderate and reversible neurotoxicity. To evaluate the hypothesis that the neurotoxicity is associated with the pharmacokinetics of active biotransformation products, we investigated the biotransformations and pharmacokinetics of oxaliplatin and ormaplatin in rats at equimolar doses. Methods: 3H-oxaliplatin and 3H-ormaplatin were administered to Wistar male rats through single bolus i.v. injections (20 μmol/kg). Blood was sampled from 3.5 min to 360 min and centrifuged at 2000 g to separate the plasma from red blood cells (RBCs). The RBCs were sonicated and centrifuged at 13 000 g to separate the cytosol from the membrane fraction. Both plasma and RBC cytosol were filtered through YMT30 membranes (Mr = 30 000 kDa), and the ultrafiltrates were analyzed using a single column HPLC technique to identify and quantitate the biotransformation products. The pharmacokinetics of oxaliplatin, ormaplatin, and their biotransformation products were characterized utilizing the curve stripping and nonlinear least-squares fitting program RSTRIP. Results: The decays of total, plasma, plasma ultrafilterable (PUF), RBC-bound, and plasma protein-bound Pt-dach (only Pt species with an intact dach carrier ligand were quantitated in this study) were described by biphasic curves. No significant kinetic differences between oxaliplatin and ormaplatin were observed for total, plasma, and PUF Pt-dach in the initial α decay phase. However, Pt-dach bound to plasma proteins fourfold more quickly for ormaplatin than for oxaliplatin, and the AUC for Pt-dach bound to plasma proteins was twofold higher for ormaplatin than for oxaliplatin. The concentration of RBC-bound Pt-dach was highest at the initial time-point of 3.5 min for both drugs, which suggested a very rapid RBC uptake. The binding of Pt-dach to RBCs was slightly greater initially for ormaplatin than for oxaliplatin. However, the RBC-bound Pt-dach decayed more rapidly for ormaplatin (t1/2αRBC = 5.1 min) than for oxaliplatin (t1/2αRBC = 15.3 min). Thus the AUCRBC was slightly greater for oxaliplatin than for ormaplatin. The AUC was also slightly greater for oxaliplatin than for ormaplatin for the Pt-dach associated with the RBC membrane and RBC cytosolic proteins. However, there was no significant difference between oxaliplatin and ormaplatin for Pt-dach in the RBC cytosolic ultrafiltrate. There was also no significant difference in the AUCpuf between oxaliplatin and ormaplatin. Both oxaliplatin and ormaplatin produced the same types of major plasma biotransformation products including Pt(dach)Cl2, Pt(dach)(Cys)2, Pt(dach)(GSH)2, Pt(dach)(GSH), Pt(dach)(Met), and free dach. The decays of oxaliplatin, ormaplatin, and their biotransformation products were described by biphasic curves. The Cmax and AUC were 19- and 15-fold higher, respectively, for oxaliplatin than for ormaplatin. However, the Cmax and AUC were 29- and 16-fold less for Pt(dach)Cl2 derived from oxaliplatin than for that derived from ormaplatin. No significant differences were observed among the Cmax values and AUC values for the other plasma biotransformation products. Pt-dach species formed in RBCs were also identified and quantitated. Oxaliplatin was observed in the RBC cytosol, while no ormaplatin was found. The same types of major RBC biotransformation products were observed including Pt(dach)Cl2, Pt(dach)(Cys)2, Pt(dach)(GSH), Pt(dach) (GSH)2, and free dach. Among these Pt-dach species, Pt(dach)Cl2 was present at a twofold lower concentration initially but persisted longer for oxaliplatin than for ormaplatin, while the other RBC biotransformation products behaved kinetically similarly and no significant AUC differences were observed. Conclusion: Our study suggests that the different toxicity and efficacy profiles between oxaliplatin and ormaplatin may be related to the different pharmacokinetic features of these two drugs, especially the different plasma concentrations of their common biotransformation product Pt(dach)Cl2. This in turn suggests that Pt(dach)Cl2 and its hydrolysis products may be uniquely neurotoxic.

Biotransformations of oxaliplatin in rat blood in vitro

The partitioning and biotransformations of oxaliplatin1 [trans‐l‐1,2‐diaminocyclohexaneoxalatoplatinum(II)] were investigated in the blood of Wistar male rats in vitro. [3‐H]‐Oxaliplatin was incubated with rat blood at 37°C in 5% CO2 and the concentrations of all Pt complexes containing the [3‐H]‐dach carrier ligand were followed for up to 12 hours. Decay for both oxaliplatin and Pt‐dach2 in the plasma ultrafiltrate (PUF) was rapid (t1/2 oxaliplatin = 0.68 h and t1/2 for Pt‐dach in the PUF = 0.85 h). After 9 hours, the concentration of oxaliplatin fell below the detection limit. By 4 hours, the PUF‐Pt‐dach reached a plateau, which was 12% of total Pt‐dach. The binding of Pt‐dach to red blood cells (RBCs) and plasma proteins was also very rapid (t1/2 RBCs = 0.58 h and t1/2 plasma proteins = 0.78 h) and reached equilibrium by 4 hours. At equilibrium, 35% of total Pt‐dach was bound to plasma proteins, 12% was in the plasma ultrafiltrate, and 53% was found associated with RBCs. Of the Pt‐dach associated with RBCs, 23% was bound to the RBC membrane, 58% was bound to RBC cytosolic proteins, and 19% was in the RBC cytosol ultrafiltrate. Thus, these studies confirm previous observations of oxaliplatin accumulation by rat RBCs. To better characterize the determinants of this accumulation, oxaliplatin and other Pt‐dach complexes were compared with respect to both their uptake by rat RBCs and their partition coefficients in octanol and water. The rank order for the rate of uptake was ormaplatin ∼ Pt(dach)Cl2 > oxaliplatin > Pt(dach)(mal); while the rank order for hydrophobicity was ormaplatin > Pt(dach)Cl2 > Pt(dach)(mal) > oxaliplatin. Thus, in general, Pt‐dach complexes appeared to be taken up better by RBCs than cisplatin or carboplatin, and the hydrophobicity of most of the Pt‐dach complexes appeared to correlate with uptake. However, factors other than the dach carrier ligand and hydrophobicity clearly influence uptake.

The Multiplicity of the D1 Dopamine Receptor

Dopaminergic neurotransmission is known to modulate a variety of behaviors, including ambulation (Ungerstedt and Arbuthnott, 1970; Pijnenburg et al., 1976), stereotyped behaviors (Creese and Iversen, 1973), self-stimulation (Phillips and Fibiger, 1973), conditioned avoidance responding (Seiden and Carlsson, 1963), stimulus control (Ho and Huang, 1975), and feeding and drinking (Ungerstedt, 1971; Fitzsimons and Setler, 1975). It is not surprising, therefore, that drugs which are believed to act primarily as dopamine receptor agonists or antagonists have important clinical utility. Our work has sought to address two questions of some neuropharmacological importance. First, what is the nature of mechanisms by which dopamine initiates many of these psychopharmacological effects, and second, is it possible to design highly specific drugs targeted only at a selected subpopulation of dopamine receptors?

High-performance liquid chromatographic separation of platinum complexes containing the cis-1,2-diaminocyclohexane carrier ligand

Platinum drugs with the 1,2-diaminocyclohexane (dach) carrier ligand have shown great promise in cancer chemotherapy, but little is known about their metabolism in the body. Since it is possible to radiolabel the dach ligand, it should be possible to quantitate the biotransformation products of these drugs, provided a method were available to separate the biotransformation products. In this paper we describe a two-column high-performance liquid chromatography system which can be used to separate many likely dach-platinum biotransformation products from the parent compounds, and allow their identification. An initial separation on a reverse-phase Partisil ODS-3 column allowed resolution of the uncharged species. The peak fractions from this column were concentrated 10-fold and reinjected onto a cation exchange Partisil 10 SCX column to allow resolution of the positively charged species. This system allowed resolution of two prototype dach-platinum drugs, (cis-1,2-diaminocyclohexane)dichloroplatinum(II) and (cis-1,2-diaminocyclohexane)malonatoplatinum(II), the aquated species likely to form from these drugs, and the complexes formed when these compounds react with glutathione, metallothionein, and amino acids. By using cation exchange chromatography at pH 2.3 as well as pH 4 and by using 14C-labeled amino acids to determine stoichiometry, it was also possible to determine the most likely structures for some of the amino acid complexes. Most importantly, this system allowed clear separation of many of the likely biotransformation products tested from the biologically important aquated species. This system should prove useful for separating and identifying the biotransformation products of dach-platinum drugs in blood and urine, in tissue culture media, and inside the cell.

Putative σ3 sites in mammalian brain have histamine H1 receptor properties: evidence from ligand binding and distribution studies with the novel H1 radioligand [3H]-(−)-trans-1-phenyl-3-aminotetralin

A novel phenylaminotetralin (PAT) radioligand, [(3)H]-(1R, 3S)-(-)-trans-1-phenyl-3-dimethylamino-1,2,3,4-tetrahydronaphthalene ([(3)H]-[-]-trans-H(2)-PAT), is shown here to label a saturable (B(max)=39+/-6 fmol/mg protein) population of sites with high affinity (K(d)=0.13+/-0.03 nM) in guinea pig brain. Consistent with previous studies which showed that PATs stimulate catecholamine (dopamine) synthesis in rat striatum, autoradiographic brain receptor mapping studies here indicate that [(3)H]-(-)-trans-H(2)-PAT-labeled sites are highly localized in catecholaminergic nerve terminal fields in hippocampus, nucleus accumbens, and striatum in guinea pig brain. Competition binding studies with a broad range of CNS receptor-active ligands and CNS radioreceptor screening assays indicate that the pharmacological binding profile of brain [(3)H]-(-)-trans-H(2)-PAT sites closely resembles histamine H(1)-type receptors. Comparative studies using the histamine H(1) antagonist radioligand, [(3)H]mepyramine, indicate that the H(1) ligand binding profile and guinea pig brain distribution of H(1) receptors and [(3)H]-(-)-trans-H(2)-PAT sites are nearly identical; moreover, both sites have about 40-fold stereoselective affinity for (-)- over (+)-trans-H(2)-PAT. These results are discussed in light of previous studies which suggested that PATs stimulate dopamine synthesis through interaction with a novel sigma-type (sigma(3)) receptor in rodent brain; it now appears instead that PATs represent a new class of ligands for brain histamine H(1) receptors that can be stereoselectively labeled with [(3)H]-(-)-trans-H(2)-PAT.

Membrane actions of male contraceptive gossypol tautomers

The role of different gossypol tautomers in the interaction of this molecule with membranes has been investigated using the isolated hemiacetal moiety of gossypol and the pH dependency of the keto-enol tautomeric equilibrium. Our results indicate that: the actions of the hemiacetal tautomer cannot explain the effects of gossypol on mitochondrial oxidative phosphorylation, lipid membrane interfacial potentials, and proton conductance of lipid bilayers; the enolate forms of gossypol are the species that bind to the membrane interface and decrease the electrostatic interfacial potential; and the uncharged (keto and/or enol) species in equilibrium with the enolate forms of gossypol give the molecule the ability to carry protons across biological membranes.