Extravascular transport of the DNA intercalator and topoisomerase poison N-[2-(Dimethylamino)ethyl]acridine-4-carboxamide (DACA): diffusion and metabolism in multicellular layers of tumor cells.

There is considerable evidence that DNA intercalating drugs fail to penetrate tumor tissue efficiently. This study used the multicellular layer (MCL) experimental model, in conjunction with computational modeling, to test the hypothesis that a DNA intercalator in phase II clinical trial, N-[2-(dimethylamino)-ethyl]acridine-4-carboxamide (DACA), has favorable extravascular transport properties. Single cell uptake and metabolism of DACA and the related but more basic aminoacridine 9-[3-(dimethylamino)propylamino]acridine (DAPA), and penetration through V79 and EMT6 MCL, were investigated by high-performance liquid chromatography. DACA was accumulated by cells to a lesser extent than DAPA and was metabolized to the previously unreported acridan by V79 but not EMT6 cells. Despite this metabolism, flux of DACA through MCL was much faster than that of DAPA. Modeling MCL transport as diffusion with reaction (metabolism and reversible binding) showed that the faster flux of DACA was due to a 3-fold higher free drug diffusion coefficient and 10-fold lower binding site density. The MCL transport parameters were used to develop a spatially resolved pharmacokinetic model for the extravascular compartment in tumors, which provided a reasonable prediction of measured average tumor concentrations from plasma pharmacokinetics in mice. Area under the curve was essentially independent of distance from blood vessels, although the combined pharmacokinetic/pharmacodynamic model predicted a modest decrease in cytotoxicity (from 1.8 to 1.1 logs of cell kill) across a 125-microm region. In conclusion, this study demonstrates that it is possible to design DNA intercalators that diffuse efficiently in tumor tissue, and that there is little impediment to DACA transport over distances required for its antitumor action.

Pharmacokinetics and metabolism of the nitrogen mustard bioreductive drug 5.

PURPOSE: To characterise the pharmacokinetics and metabolism in mice of 5-[N,N-bis(2-chloroethyl)amino]-2,4-dinitrobenzamide (SN 23862), the lead compound of a new class of bioreductive drugs in which a nitrogen mustard is activated by nitroreduction. Comparison is made with the corresponding aziridine derivative CB 1954. METHODS: Male C3H/HeN mice, bearing s.c. KHT tumours, received 3H-labelled SN 23862 or CB 1954 i.v. at 200 micromol/kg. Plasma, urine and tumour samples were assayed for total radioactivity, and for parent compounds by HPLC. Metabolites were identified by 1H-NMR and mass spectrometry. Cytotoxicity of compounds against Chinese hamster AA8 cells was determined by growth inhibition assay. RESULTS: The plasma pharmacokinetics of SN 23862 and CB 1954 were similar, with half-lives of 1.1 and 1.2 h, respectively. SN 23862 provided tumour/plasma ratios and absolute tumour AUC values almost two times higher than CB 1954. Despite this, SN 23862 was more extensively metabolised than CB 1954, the major route being sequential oxidative dechloroethylation of the nitrogen mustard moiety to the relatively non-toxic half mustard and 5-amine. The inferred chloroacetaldehyde co-product was 260 times more potent than SN 23862. A tetrahydroquinoxaline metabolite resulting from reduction of the 4-nitro group followed by intramolecular alkylation was weakly cytotoxic, while the more cytotoxic 2-amino derivative of SN 23862 was detected in trace amounts. CB 1954 was metabolised by analogous pathways, but the 4- and 2-amine nitroreduction products were the major metabolites while oxidative dealkylation was minor. CONCLUSION: The lesser propensity for SN 23862 to undergo nitroreduction in the host, relative to CB 1954, argues that dinitrobenzamide mustards may be preferable to the corresponding aziridines as bioreductive prodrugs for cancer treatment. However, the toxicological significance of oxidative metabolism of the bis(2-chloroethyl)amine moiety needs to be addressed.

Bis(dialkyl)dithiocarbamato cobalt(III) complexes of bidentate nitrogen mustards: synthesis, reduction chemistry and biological evaluation as hypoxia-selective cytotoxins.

Cobalt(III) complexes [Co(R2dtc)2(L)]+ containing two dithiocarbamate ligands (R = Me, Et, pyrrolidine) and a bidentate nitrogen mustard ligand (L) have been prepared as potential hypoxia-selective cytotoxins. The complexes were synthesized by treatment of the binuclear precursor [Co2(R2dtc)5]+ with the diamine mustards N,N'-bis(2-chloroethyl)ethylenediamine (BCE) and N,N-bis(2-chloroethyl)ethylenediamine (DCE), or their non-alkylating analogues [N,N-diethylethylenediamine (DEE) and N,N'-diethylethylenediamine (BEE)]. Cyclic voltammetry of the complexes in MeCN reveals quasi-reversible behaviour for the Co(III)/Co(II) couple, with E1/2 increasing in the order DCE < DEE approximately BCE < BEE. In MeCN/H2O electrochemical reduction is irreversible, indicating rapid substitution of H2O into the coordination sphere of the Co(II) intermediate. This fast ligand loss was confirmed by pulse radiolysis of [Co(Me2dtc)2(DEE)]+, while steady-state radiolysis showed that the initial intermediate disproportionates to [CoII(H2O)6]2+ + 2[CoII(Me2dtc)3]. The latter species reduces additional parent complex to give an overall stoichiometry of 3 mol parent complex/mol reductant. [Co(Me2dtc)2(DCE)]+ decays rapidly by an analogous mechanism in hypoxic culture medium. This reaction is not inhibited by O2, indicating that reoxidation of the Co(II) intermediate by O2 is not rapid enough to compete with ligand dissociation. The resulting free R2dtc-ligands, rather than the released mustards, are primarily responsible for growth inhibition by [Co(R2dtc)2(L)]+ complexes, although DCE release does contribute to clonogenic cell killing. Clonogenic cell killing is not appreciably enhanced under hypoxic conditions for any of the dithiocarbamato complexes. This finding, coupled with their instability in culture medium, suggests that [Co(R2dtc)2(L)]+ complexes are probably not suited for further development as bioreductive anticancer drugs.

Mechanisms of enhancement of the antitumour activity of melphalan by the tumour-blood-flow inhibitor 5,6-dimethylxanthenone-4-acetic acid.

Several studies show that the antitumour activity of melphalan (MEL) and other alkylating agents can be enhanced by the selective inhibition of tumour blood flow, although the mechanism(s) underlying these interactions are unclear. 5,6-Dimethylxanthenone-4-acetic acid (DMXAA), a new anticancer agent currently in phase I clinical trial, inhibits blood flow in murine tumours. DMXAA increased the activity of MEL against the MDAH-MCa-4 mouse mammary tumour maximally when MEL was given about 2 h after DMXAA, without compromising the maximal dose of the alkylating agent that could be given. The plasma pharmacokinetics of MEL were unchanged by DMXAA pretreatment, but the area under the concentration-time curve (AUC) for the tumour increased by 33% as a result of decreasing clearance (consistent with falling tumour blood flow). However, inhibition of tumour blood flow also leads to microenvironmental changes (e.g. acidosis and hypoxia) that might influence sensitivity to MEL. The sensitivity of KHT cells (freshly isolated from tumours) to MEL in vitro was increased by lowering of either pH or oxygen concentration (pO2), with an overall dose-modifying factor of 15 being recorded for aerobic cells at pH 7.4 versus hypoxic cells at pH 6.5. The cellular uptake of MEL by KHT cells was increased by 74% under hypoxia. Thus, DMXAA appears to augment the antitumour activity of MEL through two different mechanisms, increased exposure (via decreased tumour clearance of MEL) and increased sensitivity resulting from changes to the tumour microenvironment, both of which result from inhibition of tumour blood flow.

Hypoxia-activated prodrugs as antitumour agents: strategies for maximizing tumour cell killing.

1. Hypoxia arises in solid tumour because of inefficient blood supply. While hypoxic cells are resistant to radiotherapy and probably to many chemotherapeutic drugs they can, in principle, be turned to advantage through the development of hypoxia-activated cytotoxic drugs (bioreductive drugs). 2. Three general approaches to exploiting tumour hypoxia are discussed. The first relies on fluctuating blood flow in tumours and the consequent cycling of cells through the hypoxic compartment. The second incorporates a prodrug approach in which drug activation gives rise to cytotoxic metabolites which diffuse out of hypoxic zones. The third utilizes selective inhibitors of tumour blood flow to induce additional hypoxia and thus enhance bioreductive drug activation. 3. The latter two approaches are illustrated by recent studies with the dinitrobenzamide nitrogen mustard class of bioreductive drugs and their combination with the tumour blood flow inhibitor 5,6-dimethylxanthenone-4-acetic acid.

Menadione inhibits the alpha 1-adrenergic receptor-mediated increase in cytosolic free calcium concentration in hepatocytes by inhibiting inositol 1,4,5-trisphosphate-dependent release of calcium from intracellular stores.

In order to establish the mechanism of perturbation of hormonally regulated calcium homeostasis in hepatocytes caused by menadione, the effects of menadione on hepatic alpha 1-adrenergic receptors and on alpha 1-adrenergic receptor-mediated increase in cytosolic free calcium concentration were determined. Menadione had no detectable effect on the alpha 1-adrenergic receptor but significantly inhibited (-)-epinephrine-dependent increases in intracellular free calcium concentration in Quin2 acetoxymethyl ester-loaded hepatocytes. The hormonally induced increase in intracellular free calcium concentration is caused by formation of inositol 1,4,5-trisphosphate (IP3) which binds to a specific receptor and causes a release of intracellular ATP-dependently sequestrated calcium. The IP3-stimulated release of calcium from intracellular pools in hepatocytes was inhibited to a great extent after treatment with menadione. This inhibition could also be observed after treatment of hepatocytes with p-benzoquinone and N-ethylmaleimide and could not be reversed by the thiol-reducing reagent dithiothreitol which indicated covalent binding to an essential free sulfhydryl group. The inhibition of IP3-dependent release of intracellular calcium was accompanied by a large increase in the number of detectable IP3 receptors without any change in the dissociation constant as determined in permeabilized hepatocytes. The increase in IP3 receptors caused by menadione could be reversed by dithiothreitol which suggests the involvement of free sulfhydryl groups. It is concluded that the IP3 receptor plays an important role in the mechanism of menadione-induced perturbation of hormonally regulated calcium homeostasis in rat hepatocytes.

Changes in inositol-1,4,5-trisphosphate binding to hepatic plasma membranes caused by temperature, N-ethylmaleimide and menadione.

We investigated the effects of the sulfhydryl-alkylating agent N-ethylmaleimide (NEM) and menadione--a sulfhydryl-arylating agent, which can undergo redox cycling--on the [3H]inositol-1,4,5-trisphosphate ([3H]IP3) binding properties of rat plasma membranes. Rat liver plasma membranes were incubated for 15 min at 37 degrees with 0.1 mM, 0.2 mM, 0.5 mM NEM or 0.3 mM menadione and subsequently diluted for use in [3H]IP3 binding studies. An incubation as such (15 min at 37 degrees) already caused the dissociation constant (Kd) of [3H]IP3 binding to increase from 1.9 +/- 0.2 nM to 3.4 +/- 0.2 nM, with only a small change in the maximal number of IP3 binding sites (Bmax-values of 401 +/- 32 and 349 +/- 13 fmol/mg protein, respectively). Incubation with NEM (0.1, 0.2 and 0.5 mM) resulted in a dose dependent decrease in the Bmax with 41, 87 and 99%, respectively, without a significant change in the Kd compared to the time matched controls. Menadione (0.3 mM) decreased the Bmax with 54% without affecting the Kd. In contrast to our findings at 37 degrees, incubation of the plasma membranes with NEM (0.5 mM) at 0 degrees for 30 min did not affect [3H]IP3 binding. In order to account for this discrepancy, the reaction rate of NEM with glutathione was examined at both 0 degrees and 37 degrees by recording the changes in the UV-spectrum of NEM (lambda max = 302 nm) after addition of 1 mM NEM to 1 mM glutathione. A similar reaction rate was observed at both temperatures. These data suggest that alkylation of a sulfhydryl-moiety in the IP3-receptor molecule causes inactivation of the receptor function. Since at 0 degrees NEM is still able to react with sulfhydryl groups, but not able to inactivate the IP3-receptor, it can be suggested that the sulfhydryl-moiety of the IP3-receptor is masked and cannot be reached by a sulfhydryl-alkylating agent at 0 degrees.

The effects of radical stress and N-ethylmaleimide on rat hepatic alpha 1-adrenergic receptors.

Liver plasma membranes of non-treated rats or rats treated with carbon tetrachloride were prepared. Liver plasma membranes obtained from non-treated rats were preincubated with cumene hydroperoxide, N-ethylmaleimide or hydrogen peroxide in the absence or presence of Fe2+. The effects of in-vivo or in-vitro treatment on hepatic alpha 1-adrenergic receptors were assessed in a [3H]prazosin binding assay. All treatments except preincubation with hydrogen peroxide alone caused a decrease in the number of alpha 1-adrenergic receptors without a change in the affinity of [3H]prazosin. The decrease in receptor number was accompanied by an increase in the level of lipid peroxidation in the plasma membranes. The results indicate that the hepatic alpha 1-adrenergic receptor is vulnerable to free radical stress.