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.

Comparison of aromatic and tertiary amine N-oxides of acridine DNA intercalators as bioreductive drugs. Cytotoxicity, DNA binding, cellular uptake, and metabolism.

Some N-oxide derivatives of DNA intercalators are bioreductive prodrugs that are selectively toxic under hypoxic conditions. The hypoxic selectivity is considered to result from an increase in DNA binding affinity when the N-oxide moiety is reduced. This study investigated whether differences in DNA binding affinity between N-oxides and their corresponding amines, measured by equilibrium dialysis, can account for the hypoxic cytotoxicity ratios (HCR) of tertiary amine N-oxide (-tO) and aromatic N-oxide (-aO) derivatives of the 1-nitroacridine nitracrine (NC) and its non-nitro analogue 9-[3-(N,N-dimethylamino)propylamino]acridine (DAPA). Cytotoxicity was measured in aerobic and hypoxic suspensions of Chinese hamster ovary (CHO) AA8 cells by clonogenic assay. HCR were much greater for NC-tO (820-fold) than for NC (5-fold) or NC-aO (4-fold), whereas DAPA and its N-oxides lacked hypoxic selectivity (1-fold). DNA binding measurements demonstrated that binding affinity is lowered more by aromatic than tertiary amine (side-chain) N-oxides, an observation that does not correlate with HCR. Compounds were accumulated in cells to high concentrations (C(i)/C(e) approximately 10-200), with the exception of the tertiary amine N-oxides, for which the ratio of intracellular to extracellular drug was less than unity. For NC-tO this probably resulted from low pK(a) values for both the acridine chromophore and the side-chain, whereas DAPA-tO may be too hydrophilic for efficient membrane permeation. Bioreductive drug metabolism, assessed by HPLC, was faster for the NC than the DAPA N-oxides. The high HCR of NC-tO relative to NC-aO is ascribed to the rapid and selective reduction of its N-oxide moiety, followed by activation of the NC intermediate by O(2)-sensitive reduction of its 1-nitro group to the corresponding 1-amine. The metabolism studies suggest that unmasking of DNA binding affinity by reductive removal of the N-oxide moiety, although not the only determinant, is important and needs to occur before nitroreduction for optimal effect.

Extravascular diffusion of tirapazamine: effect of metabolic consumption assessed using the multicellular layer model.

PURPOSE: Hypoxia-selective cytotoxic agents, like tirapazamine (TPZ), must diffuse considerable distances in tumors to reach their target cell population. This study uses a new three-dimensional tissue culture model, in which cells are grown as multicellular layers (MCL), to investigate whether metabolic consumption of TPZ is sufficiently rapid to compromise its extravascular diffusion in tumors. METHODS AND MATERIALS: V79-171b and MGH-U1 cells were grown as MCL to thicknesses of approximately 120 and 360 microm respectively. The extent of hypoxia in MCL, as assessed by EF5 binding, was modulated by altering gas-phase O2 content, and flux of TPZ through MCL was investigated by high-performance liquid chromatography (HPLC). Data were fitted to a diffusion-reaction mathematical model to determine the diffusion coefficient of TPZ in the MCL (DM) and the rate of its metabolic consumption under anoxia. These parameters were used to simulate TPZ transport in tumors. RESULTS: The flux of TPZ through well-oxygenated MCL (equilibrated with 95% O2) was well fitted as Fickian diffusion without reaction, with a D(M) of 7.4 x 10(-7) cm2s(-1) (12-fold lower than in culture medium) for V79 and 1.3 x 10(-6) cm2s(-1) for MGH-U1 MCL. Flux of TPZ was suppressed under anoxia, and fitting the data required inclusion of a reaction term with a rate constant for metabolic consumption of TPZ of 0.52 min(-1) for V79 and 0.31 min(-1) for MGH-U1 MCL. These transport parameters would translate into a 43% or 30% decrease respectively in TPZ exposure, as a result of drug metabolism, in the center of a slab of anoxic tissue 100 microm in thickness. CONCLUSIONS: MCL cultures provide an in vitro model for investigating the interaction between metabolic consumption and diffusion of bioreductive drugs. If rates of diffusion and metabolism similar to those measured in V79 and MGH-U1 MCL apply in tumors, then cells in large confluent regions of hypoxia would be partially protected by failure of TPZ penetration. Simulation of extravascular transport of TPZ-like bioreductive drugs demonstrates that the optimum metabolic rate constant is determined by two competing requirements: it should be high enough to ensure potent cytotoxicity under hypoxia, yet low enough that penetration is not severely compromised.

An experimental and mathematical model for the extravascular transport of a DNA intercalator in tumours.

A new in vitro model has been developed for investigating extravascular diffusion of therapeutic agents in tumour tissue. V79-171b or EMT6/Ak cells are grown on porous Teflon support membranes and submerged in a large reservoir of medium, to give diffusion-limited 'multicellular membranes' (MMs) c. 200 microm in thickness. MMs are histologically similar to multicellular spheroids, but their planar rather than spherical geometry facilitates direct measurement of the flux of radiolabelled agents through the multicellular structure. For [14C]urea, flux kinetics through V79-171b MMs was modelled as simple diffusion, yielding a diffusion coefficient in the MM (DMM) of 1.45 x 10(-6) cm2 s(-1), 11-fold lower than in culture medium. Flux of the 3H-labelled DNA intercalator 9-[3-(N,N-dimethylamino)propylamino]acridine (DAPA) was dramatically slower than urea. Modelling this over the first 5 h gave a DMM of 1.3 x 10(-8) cm2 s(-1), but over longer times the kinetics was not consistent with simple diffusion. Flux of DAPA was markedly increased in the presence of 50 mM ammonium chloride, indicating that sequestration in acidic endosomes is a major impediment to flux. Accumulation in cytoplasmic vesicles was confirmed by fluorescence microscopy. The DAPA flux kinetics, with and without ammonium chloride, was well fitted by a reaction-diffusion model with reversible cellular uptake (modelled as binding), using uptake parameters determined in separate experiments with V79-171b single-cell suspensions. This study demonstrates the utility of the MM model for determining extravascular transport parameters, and indicates that much of the impediment to diffusion of basic DNA intercalators in tumour tissue may arise from lysosomal sequestration rather than DNA binding.

Multicellular membranes as an in vitro model for extravascular diffusion in tumours.

Efficient extravascular diffusion is a critical requirement for hypoxic cell radiosensitisers, bioreductive drugs and hypoxic cell markers that must reach cells distant from functional blood vessels in tumours. The diffusion of simple nitroimidazoles with neutral (misonidazole, miso) and basic (pimonidazole, pimo) side chains, as well as 2-nitroimidazoles with acridine (NLA-1) and phenanthridine (2-NLP-3) DNA intercalating moieties was investigated using multicellular membranes (MMs), a new in vitro model for the extravascular compartment of tumours. The diffusion of miso through V79 MMs was concentration independent over the range 0.1-10 mM. Mathematical modelling of the flux kinetics provided a diffusion coefficient in MMs (DMM) of 5.5 x 10(-7) cm2 s-1 which was approximately 13-fold lower than in culture medium. Flux was little affected by the extent of hypoxia in MMs, indicating that hypoxic metabolism does not compromise diffusion of miso over distances in the order of 200 microns. The DMM for pimo was similar to miso, while those for 2-NLP-3 and NLA-1 were both lower. The results demonstrate compromised extravascular diffusion for DNA-intercalating nitroimidazoles, but indicate that this problem is more severe for the basic acridine derivative, NLA-1, than for the phenanthridine, 2-NLP-3. The MM model appears to be well suited to quantitative determination of drug diffusion in a multicellular environment.

A simple method for estimating tubular and net glomerular capillary pressures in the isolated perfused kidney.

To estimate glomerular filter resistance it is necessary to measure net glomerular capillary pressure, tubular pressure, and glomerular filtration rate. Net glomerular capillary pressure is defined as the difference between glomerular capillary hydrostatic and oncotic pressures. A method has been developed for estimating both tubular pressure and net glomerular capillary pressure for the whole kidney from the time course of the rise in ureteral pressure on ureteral occlusion. Pressure and flow relations which apply in the kidney during ureteral occlusion have been represented by a simple mathematical model. Estimates of tubular pressure and net glomerular capillary pressure are made by representing the first 60s of the rise in ureteral pressure as the linear combination of two exponential terms. These estimates compare favourably with corresponding results obtained from a more general numerical analysis. Direct measurements of tubular pressure and net glomerular capillary pressure made by micropuncture of early proximal tubules have been compared with analytical estimates from subsequent ureteral occlusion. There was no significant difference between results obtained using either method for a wide range of perfusion pressures.