Greater Lipid Solubility Research Articles

Chloride Transport Across Planar Lipid Bilayers and Cell Membranes by Steriod-Based Synthetic Anion Transporters

Cholapods are steroid-based synthetic anion transporters derived from cholic acid that bind anions with high affinity and promote their efflux from liposomes. To understand better cholapod-mediated anion transport, we studied the cholapods AS09, LJ09 and TL145 in planar lipid bilayers, single cells and polarised epithelia. When compared with AS09, LJ09 has higher anion affinity, whereas TL145 has greater lipid solubility. In planar lipid bilayers, cholapod-mediated currents relaxed to a steady-state after an initial peak when membrane voltage was stepped. Anion transport by cholapods was concentration-dependent with anion conductance decreasing with increasing anionic radius. Evaluation of the rate constants for anion transport by cholapods using the method of Lauger (Science 1972;178:24-30) suggested that movement of the anion-transporter complex across the membrane is the rate-limiting step of cholapod-mediated anion transport. To investigate cholapod-mediated anion transport in single cells, we used Fischer rat thyroid epithelial cells stably expressing the halide-sensitive yellow fluorescent protein (FRT-YFP) (gift from A Verkman, UCSF) to monitor anion influx. In the absence of cholapod, addition of iodide (10 mM) to FRT-YFP cells had little or no effect on fluorescence. However, in the presence of cholapod, YFP fluorescence decreased following iodide influx into FRT-YFP cells and when iodide was washed from the external solution fluorescence quenching reversed. Finally, to evaluate cholapod-mediated transport at the tissue level, we grew FRT-YFP cells as polarised epithelia and measured transepithelial Cl- movement in Ussing chambers. Treatment of FRT-YFP epithelia with cholapods led to the development of transepithelial Cl- current that was absent in untreated epithelia. Because TL145 was the most effective transporter in all assays, we conclude that lipid solubility is an important determinant of transmembrane anion transport by the cholapod family of synthetic anion transporters. [Supported by the EPSRC].

Rational use of medicines

A PHYSICIAN treating people usually has information available from randomised controlled trials to allow rational, evidence-based prescribing. Partly because of the cost of clinical trials and partly because many of...

Frequency-Dependent Block in Humans?

In Response: We concur with Stevens that, as our study demonstrated, a frequency- dependent action of local anesthetics is unlikely to be a mechanism at work in stimulation-induced (i.e., muscular exercise) reductions in latency of onset of sensory or motor block following peripheral nerve injections in humans.1,2 In fact, frequency-dependent block (use-dependent; phasic block) is likely to be limited clinically in humans to cardiac tissue exposed to critical concentrations of tertiary local anesthetics. Experimentally, sodium conductance decreases with repetitive neural stimulation. Invitro studies in mammalian peripheral nerves and in cardiac tissue have demonstrated that the intensity or depth of impulse blockade increased as the rate of stimulation increased.3,4 In reptilian sciatic nerve, a marked reduction in the height of the tenth action potential when compared with the first has been shown after exposure to local anesthetics. This phenomenon was dependent upon the agent used and the frequency of stimulation,5 and was maximal as early as the third, and as late as the twentieth impulse. All tertiary local anesthetic agents demonstrated this effect at a stimulation frequency of 40 Hz.6 Being highly lipid soluble, bupivacaine is a logical choice for studying frequency-dependent block in humans.1 While greater lipid solubility has been associated with increased repetitive stimulation requirements to reach maximal blocking effects compared with less lipid-soluble drugs such as lidocaine,3 much greater degrees of frequency-dependent cumulative block occur after bupivacaine than after lidocaine at lower stimulation frequencies.6 Frequency-dependent block persists for as long as the train of neural impulses that drives depolarization continues, and is ideally studied in a contracting heart responding to spontaneous nodal activity. Since the tonic impulse traffic in peripheral nerves differs from that observed in the heart, even when employing 5 min of continual muscular exercise as was done in our study,1 it is logical that, as Stevens noted, the enhancement of local anesthetic action by employing a frequency-dependent block vanishes within seconds after peripheral nerve blocks.2 With the recent renewed attention being given to cardiac toxicity of local anesthetics, however, there continues to be a necessity to study frequency-dependent block induced by local anesthetic drugs applied to contracting muscle. Frequency-dependent block is likely responsible for the observed cardiac toxicity of bupivacaine, its resistance to conventional pharmacological treatments, and its propagation for extended periods of time while cardiac activity persists. Frequency-dependent block is thus not necessarily species-dependent, but more appropriately should be characterized as being organ or system-dependent. Kenneth D. Candido, MD Department of Anesthesiology University of Illinois College of Medicine Chicago, IL [email protected] Kenneth D. Langen, MD Michael King, MD Guido Marra, MD Department of Orthopaedic Surgery Loyola University Medical Center Maywood, IL Alon P. Winnie, MD Northwestern University/Feinberg School of Medicine Chicago, IL

A Comparison of Single-Dose Caudal Clonidine, Morphine, or Hydromorphone Combined with Ropivacaine in Pediatric Patients Undergoing Ureteral Reimplantation

Caudal blockade is a common technique for pediatric postoperative analgesia. While safe and effective, caudal opioids are associated with troublesome side effects. Caudal clonidine may offer significant analgesic benefits. We prospectively compared the analgesic, side effect, and rehabilitation profiles of caudal clonidine, hydromorphone, or morphine in a group of 60 pediatric patients undergoing ureteral reimplantation. Patients aged 6 mo to 6 yr were evenly and randomly enrolled in a double-blind manner. Patients received a single caudal dose of 2 mcg/kg of clonidine, 10 mcg/kg of hydromorphone, or 50 mcg/kg of morphine, combined with 1.0 mL/kg of 0.2% ropivacaine with epinephrine. After sevoflurane in oxygen/air anesthesia, all subjects received proxy nurse-controlled analgesia with morphine. Postoperative pain intensity, use of IV morphine, and side effects were assessed during the first 24 h. Oral intake and discharge home were recorded. Caudal clonidine resulted in less postoperative nausea and vomiting (P = 0.01) and pruritus (P = 0.007) than did caudal hydromorphone or caudal morphine. Caudal morphine produced more sustained initial analgesia than did caudal clonidine (P = 0.02). No difference was observed in pain scores, total morphine use, time to first oral intake or discharge home. No postoperative respiratory depression, excessive sedation, hypotension, or bradycardia was identified. Although caudal morphine may result in more sustained initial analgesia, caudal clonidine combined with nurse-controlled analgesia appears to provide comparable analgesia with fewer side effects. Based on these results, the use of caudal clonidine may be superior to caudal opioids after pediatric ureteral reimplantation.

LIPID SOLUBILITY OF PANCURONIUM AND VECURONIUM DETERMINED BY N-OCTANOL/WATER PARTITIONING

We have measured the n-octanol/water distribution coefficients at 37 degrees C of two steroidal myoneural blockers (pancuronium and vecuronium) and of their 3-hydroxy metabolites over a pH range from 1.5 to 11. Pancuronium was found not to be lipid soluble. The mean distribution coefficient of 3-hydroxy vecuronium at pH = 7.4 was 0 and that of vecuronium 0.12 (SD 0.1). Distribution coefficients of the parent drug showed a slightly greater lipid solubility than the metabolite (11.37 (2.91) and 9.53 (3.44), respectively (mean t0.05 x SE)). Because of its lipid solubility, vecuronium may penetrate into lipid structures.

Peracetic acid sterilization: a timely development for a busy healthcare industry.

Peracetic acid is a component of an equilibrium mixture that includes acetic acid, hydrogen peroxide, and water. Although it was first reported in 1902, not until years later did Greenspan and MacKellar find it to be bactericidal at 0.001%, fungicidal at 0.003%, and sporicidal at 0.30%.’ There are no specific data explaining how peracetic acid actually destroys the cell; it has been speculated that it functions like other peroxides and oxidizing agents. Block suggests that it destroys the chemosmotic function of the lipoprotein cytoplasmic membrane through dislocation or rupture of the cell wall.Peracetic acid is sporicidal at low temperatures, and it remains effective in the presence of some organic material. Sprossig stated that peracetic acid has advantages for disinfection and sterilization not found in any other agent. Block states that, “It would be desirable to have a chemical with the attributes of hydrogen peroxide-effective germicidal and sterilizing capabilities, no harmful decomposition products, and infinite water solubility-but with greater lipid solubility and freedom from deactivation by catalases and peroxidases. Such a compound exists. It is peracetic acid.” Fraser adds that, “A prime advantage of peracetic acid is that it is a nonfoaming, water-soluble liquid that is fast acting and nonderivatising, as well as environmentally accepted.”

Glycine Conjugation of the Substituted Benzoic Acid in Mice: Structure-Metabolism Relationship Study II.

Glycine conjugation of a series of substituted benzoic acids was investigated in the mouse liver and kidney mitochondria. Correlations between the structure of 24 substituted benzoic acids and glycine conjugation were obtained. The extent of glycine conjugation of a series of substituted benzoic acids in liver mitochondria was different from that in kidney mitochondria. Glycine conjugation increased with greater lipid solubility in the mouse liver and kidney. The steric effect of the substituent had a far greater influence over the glycine conjugation in kidney, while the size of the substituent played a small role in the pattern of conjugation in liver. The formation of the glycine conjugate in liver was also dependent on the substituent electronegativity.

Glycine conjugation of the substituted benzoic acids in vitro: Structure-metabolism relationship study.

The relationships between the chemical structure and glycine conjugation of 10 para- and 8 meta-monosubstituted, and 6 disubstituted benzoic acids were examined in the rat liver and kidney mitochondria. For the simultaneous determination of the acid and its glycine conjugate, a simple and specific high performance liquid chromatographic method was developed. The extent of glycine conjugation of a series of substituted benzoic acids in liver mitochondria was similar to that in kidney mitochondria. Glycine conjugation increased with greater lipid solubility. On the other hand, more bulky substituents at the p- or m-position on the benzene ring reduced the glycine conjugation. The dependence on van der Waals volume (Vw) values for substituents reflects the importance of steric effects in the glycine conjugation. However the steric effect of the substituent was slightly less pronounced at the m-position than at the p-position. These results indicate that an active site of the enzyme possess limited steric bulk tolerance.

Arteriolar smooth muscle responses are modulated by an intramural diffusion barrier

Arterioles (40-80 micron diameter) were isolated from the hamster cheek pouch, cannulated at both ends, and perfused with 3-(N-morpholino)propanesulfonic acid (MOPS)-buffered physiological salt solution (PSS). The vessels were observed with an inverted microscope and video system, and arteriolar diameter was measured. Arterioles were found to be 100 times more responsive to the alpha 1-adrenoceptor agonist phenylephrine when applied to the adventitial surface than when applied to the luminal surface. In contrast, SKF 89748-A, also an alpha 1-adrenoceptor selective agonist, but with a much greater lipid solubility than phenylephrine, was equipotent from either surface of the arteriole. We hypothesized that the difference between the two drugs was due to the ability of SKF 89748-A to permeate a diffusion barrier in the arteriolar wall because of its lipid-solubility. To test this hypothesis, a spectrum of antagonists with different sites of action and lipid solubilities was tested. The alpha-adrenoceptor antagonists phentolamine and benextramine and the muscarinic receptor antagonists atropine, scopolamine, and methscopolamine were all found to be more potent at blocking the action of appropriate agonists when applied to the same surface of the arteriole as the agonist than when applied to the opposite surface. Octanol-water partition coefficients were measured for each of the compounds, and these were found to be highly correlated with the ratio of luminal potency to adventitial potency for each of the drugs tested. These data support the hypothesis that the endothelial cell layer in these arterioles forms a barrier to the diffusion of small, water-soluble molecules from the lumen to the smooth muscle cell layer. Such a barrier may have a significant effect on arteriolar reactivity.

Cardiovascular responses to metipranolol and timolol eyedrops in healthy volunteers.

1. Intraocular pressure and cardiovascular responses to metipranolol 0.1% and 0.3% and timolol 0.25% eyedrops were measured in a balanced single dose placebo-controlled crossover study in eight healthy volunteers aged 34-58 years. 2. Timolol 0.25% and metipranolol 0.3% reduced intraocular pressure throughout the 6 h period of observation to a similar extent. Metipranolol 0.1% was marginally less effective, significantly reducing pressure up to 4 h only. 3. No drug treatment significantly altered resting heart rate or blood pressure. Timolol 0.25% significantly reduced exercise tachycardia (P less than 0.05), an effect which was not shown by metipranolol 0.1 or 0.3%. Exertional pain in the legs occurred more frequently after timolol 0.25% and metipranolol 0.3% than after metipranolol 0.1% or placebo eyedrops. 4. Octan-1-ol/pH 7.4 buffer distribution coefficients at 37 degrees C were found to be: metipranolol 5.19, timolol 0.84, indicating that metipranolol has an approximately 6-fold greater lipid solubility. 5. It is concluded that, by comparison with timolol, metipranolol in eyedrop concentrations up to 0.3%, despite its greater lipid solubility, reaches concentrations in the systemic circulation which are less likely to affect the heart.

Placental transfer of anticonvulsants in the rabbit: the effect of maternal protein binding and fetal flow

The fetal-to-maternal ratios of carbamazepine, antipyrine and phenytoin are principally determined by maternal protein binding, though greater lipid solubility may enhance the transfer of valproate compared to that of other drugs at high flows. Placental clearance of all anticonvulsants showed flow-dependent characteristics. This is in line with our findings for basic drugs.

The therapeutic use of heroin: a review of the pharmacological literature

Heroin is currently being advocated by some as a superior therapeutic agent for use in terminal illness. However, a review of the literature on heroin presently available does not support this contention. Administered orally, heroin is approximately 1.5 times more potent than morphine in controlling chronic pain in terminal cancer patients. Its effects on mood and the incidence and nature of side effects do not differ from those of morphine except in males where poorer pain control probably accounts for the worse effect on mood. Given parenterally for acute pain, heroin is 2-4 times more potent than morphine and faster in onset of action. When the potency difference is accounted for, the pharmacological effects of heroin do not differ appreciably from those of morphine. Heroin is metabolized to 6-acetylmorphine and morphine. After oral administration of heroin, morphine but not heroin or 6-acetylmorphine is detected in blood. In this case, heroin is a prodrug for the delivery of systemic morphine. Following acute i.v. administration, heroin appears transiently in blood with a half-life of about 3 min. The half-life of heroin exposed to blood or serum in vitro is 9-22 min, indicating that organ metabolism is involved in blood clearance as well. Direct renal clearance of heroin is less than 1% of the administered dose. In animal studies, heroin and 6-acetylmorphine are both more potent and faster acting than morphine as analgesics, effects attributed to their greater lipid solubility and subsequent penetration of the blood-brain barrier. Given centrally, morphine is more potent than heroin and 6-acetylmorphine in producing analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)

1359 BILIRUBIN (BR) FLUX AND DISTRIBUTION IN THE BRAIN

Carotid artery injection of Evans Blue or albumin-bound bilirubin (ALB+BR) for 60 sec. in adult rats produced no cortical staining with an intact blood-brain barrier(BBB) and non-uniform “tufted” perivascular staining after barrier opening to ALB by 1.8 Molar Urea × 30 sec. Unbound BR given for 10-60 sec. to 25 rats without barrier opening, followed by carotid perfusion with saline, produced uniform staining, and the uptake of BR measured by chloroform extraction was dose-dependent (r=0.641, p<0.001). Retained BR was 11.6±6.2% (MEAN±S.D.) of the dose injected, and the permeability-surface area (P.S) product for BR was 0.13±0.05 ml·min−1 · g−1. The estimated volume of distribution for unbound BR at 60 sec. was 195±72 μl/g. When injection of unbound BR was followed by re-infusion with 5% ALB, residual brain BR decreased as shown:Bound BR crossing a disrupted BBB and unbound BR crossing an intact barrier appear to have different extravascular patterns of distribution, consistent with the greater lipid solubility and capillary permeability of unbound BR, versus the tendency of ALB + BR to cross the barrier only at osmotically opened capillaries. Because ALB decreases residual brain BR, we speculate that BR is initially distributed in a “shallow” compartment and is not immediately strongly bound or irreversibly precipitated in the brain.

Benthic-pelagic coupling in sewage-affected marine ecosystems

Coupling between degraded benthic habitats and populations of epibenthic and pelagic organisms is evident around major sewage outfalls in the Southern California Bight. The distribution of bottom-feeding fishes appears to be affected, but sampling has not been intensive enough to rigorously define patterns for more than a few species. Some evidence suggests that changes in fish abundance may be related to changes in the distribution and abundance of benthic prey; those species (e.g. Dover sole) that can adapt their diets to take advantage of dense populations of opportunistic polychaetes may be found in large numbers around outfalls. Some benthic crustacean feeders are at a disadvantage. Sewage effluent is an allochthnous source of organic matter to the food web and Dover sole and ridgeback shrimp both show shifts in stable isotope ratios consistent with the thesis that sewage-derived organic matter in sediments is trophically transferred to epibenthic organisms. Constructing a carbon budget based on the δ 13C shifts in these species from area of a major outfall relative to a distant control station suggests that there is an in situ source of isotopically very light carbon in outfall sediments, perhaps from CO 2 fixation by chemoautotrophs and biological methane oxidation. Benthic-pelagic coupling is also evident in the accumulation of chlorinated hydrocarbons by fish and other organisms. At the two major Los Angeles area outfalls log-transformed liver-to-sediment concentration ratios of chlorinated benzenes, which are accumulated to moderate levels in fish, show a linear relationship to the log of K ow (the octanol-to-water partioning coefficient) of these compounds. Chlorinated hydrocarbons with greater lipid solubility, e.g. PCBs and p,p-DDE, are concentrated to very high levels. Studies of food webs using Cs/K as a measure of trophic level suggest that trophic-step magnification is concentrating these compounds in higher trophic levels.

Attempt at pharmacological differentiation of central β-adrenergic receptors

The β-adrenergic stimulants isoprenaline, salbutamol and clenbuterol decreased motor activity, reduced the interest of fasting mice in food pellets (‘Tantale’ test), and antagonized high-dose apomorphine hypothermia. Clenbuterol was 16–60 times more potent than the other two agonists in all tests, apparently because of better crossing into the C.N.S. The β-adrenergic blockers, d,1-propranolol and 1-penbutolol, completely antagonized the effects of salbutamol and clenbuterol in tests of motor activity and apomorphine hypothermia, penbutolol being 32–60 times more active than propranolol, perharps due to its greater lipid solubility. The Tantale test allows unabigous differentiation between penbutolol and propranolol. While the first completely blocked the effects of clenbuterol and salbutamol, the second was effective only within a narrow dose range. Finally, the dose-effect relationships of the 3 agonists were parallel in the Tantale test, while they were divergent in the other two. This suggeststhat different classes of β-adrenergic receptors may be involved in the various test.

A COMPARATIVE STUDY OF THE EFFECTS OF ( + )-CATECHIN AND 3-PALMITOYL-( + )-CATECHIN ON ALCOHOLIC FATTY LIVER IN THE RAT

Administration of a liquid diet containing 36% of the total calories as ethanol to male Wistar rats for three weeks caused a threefold increase in total hepatic lipid, a tenfold increase in hepatic triglycerides, a twofold increase in hepatic unesterfied fatty acids, and elevated the serum activities of the liver enzymes alanine aminotransferase and glutamate dehydrogenase, when compared with control data from animals pair-fed an isocaloric amount of glucose instead of ethanol. Simultaneous administration of either ( + )-catechin (200mg/kg by gastric tube) or 3-palmitoyl-( + )-catechin (l00mg/kg daily by gastric tube) to ethanol treated animals caused significant reductions in the extent of fat accumulation and hepatocellular damage as assessed by both biochemical and histological means. 3-Palmitoyl-( + )-catechin was more effective than ( + )-catechin, even though it was administered at only half the dose of that of the former. Pharmacokinetic studies in which [U-14C] labelled 3-palmitoyl-( +)-catechin and (+ )-catechin were given orally (150mg/kg each) to male rats showed that 3-palmitoyl-( + )-catechin achieved a higher peak tissue concentration and remained in the liver for a longer period than (+ )-catechin. These properties of 3-palmitoyl-( + )-catechin are thought to be due to its greater lipid solubility, thereby explaining its greater potency in the chronic fatty liver studies.

Distribution of cardiac glycosides in heart and brain of dogs and their affinity to the (Na++K+)-ATPase

The tissue distribution after repeated intravenous administration of tritium-labelled digoxin, β-methyldigoxin and ouabain was examined in heart and brain of 6 beagle dogs. In addition, the (Na++K+)-ATPase activity was measured in various heart and brain areas, and its affinity to the cardiac glycosides was determined. The glycoside concentrations in the atria are lower than in the ventricles, and the left heart areas show higher concentrations than the right areas. Significant differences in the (Na++K+)-ATPase activity or its binding capacity in the various heart areas, which could be responsible for this characteristic distribution pattern, were not found. In agreement with its greater lipid-solubility, β-methyldigoxin shows a higher accumulation in the brain than digoxin and ouabain. However, while β-methyldigoxin is evenly distributed throughout all brain areas, concentration differences are found for digoxin and ouabain in the telencephalon, cerebellum and brain stem. This characteristic distribution of the more polar glycosides may be partly determined by the different structure of the capillaries in the central nervous system. In addition, the binding affinities for digoxin and ouabain also differ in the various crude brain preparations. In the diencephalon, pons, cerebellum and medulla the dissociation constants as a reciprocal measure of the binding affinity were lower for digoxin with 7.5 to 9.9×10−9 than in the telencephalon, mesencephalon and spinal cord with dissociation constants of 1.1 to 1.45×10−8 M. Since, in these brain areas higher glycoside concentrations per g wet weight were also measured, the glycoside accumulation in the various brain areas could be dependent on the higher receptor affinity of these brain areas. On the other hand, the binding affinities for β-methyldigoxin were the same in all brain areas, with a mean dissociation constant of 1.45×10−8 M.

Tetracycline-induced inhibition of Na+ transport in the toad urinary bladder.

The effects of three tetracyclines, demethylchlortetracycline (DMC), minocycline (MNC), and oxytetracycline (OTC), on Na+ transport (measured as short-circuit current) were examined in toad urinary bladders mounted in modified Ussing chambers. During a 1-h incubation period serosal DMC (but not MNC or OTC) inhibited basal Na+ transport, whereas MNC (but not DMC or OTC) inhibited ADH-stimulated Na+ transport. MNC also inhibited cyclic AMP-stimulated Na+ transport. During longer incubation periods all three drugs inhibited basal Na+ transport. The DMC-induced inhibition of basal Na+ transport and the MNC-induced inhibition of ADH-stimulated Na+ transport were paralleled by an inhibition of the active conductance of the bladders. Thus, although all three drugs inhibit basal Na+ transport, only MNC inhibits ADH-stimulated Na+ transport. This effect does not correlate with the known effects of the tetracyclines on ADH-stimulated water flow or with drug-protein binding, and may be related to the greater lipid solubility of MNC.

Pharmacokinetics.Selectivity of action related to physicochemical properties and kinetic patterns of anticancer drugs

Since the therapeutic index of drugs used for the treatment of cancer is small, optimal use depends on whatever advantage can be gained by achieving an effective concentration at the critical site in the cancer cell for a period of time sufficient to kill that cell while minimizing the action on normal cells or allowing their recovery. Advances in 1) methodology for measurement of minute amounts of drugs in tissues and body fluids, 2) better understanding of cell cycle kinetics and 3) development of kinetic models and computer simulation of compartmental drug distribution now aid better choice of dosage, mode of administration and treatment schedules. Limitations on the entry of amethopterin (Methotrexate) into cells and body compartments is compared with another folate antagonist, metroprine (DDMP), having the same mode of action, yet able to penetrate into brain. CSF and amethopterin-resistant cells because of greater lipid solubility. Pharmacokinetic considerations related to the limited effectiveness of present drugs for the treatment of brain tumors lead to the concept of "compartmental chemotherapy" based on selective drug contact with tumor combined with selective protection of tissues of limiting toxicity.

Pharmacokinetics of the fluoropyrimidines: implications for their clinical use

Summary There are four areas of current research interest in the fluoropyrimidines which may improve clinical utility of this class of drugs (Table 2) . First, there is the obvious need for comprehensive pharmacokinetic information in man not only on the parent drug, be it 5FU, FUdR or ftorafur, but also on all potentially active metabolites. Second, knowledge of the distribution of uridine kinase, phosphoribosyl transferase and thymidine kinase in human bone marrow, gastrointestinal mucosa and accessible tumors would help clarify which of these enzymes controls 5-FU activation. Third, the availability of techniques to determine FdUMP formation within the target tumor cell provides an opportunity for predictive pharmacologic testing which should be fully exploited, especially in the adjuvant situation where tumor tissue is available and severe drug toxicity unacceptable. Table 2 . Major areas of current interest 1. Adequate pharmacodynamic studies in man of parent drug and active metabolites. 2. Distribution ofuridine kinase, phosphoribosyl transferase and thymidine kinase in normal and malignant human tissues. 3. Predictive pharmacologic testing based on either direct measurement of FdUMP formation or levels of nucleotide synthetic enzymes in tumour samples. 4. Synthesis of additional analogs which share the lipid solubility of ftorafur and its reduced marrow toxicity, but which lack its neurotoxicity. Finally, the greater lipid solubility and lessened marrow toxicity of ftorafur represents an improvement over 5-FU which may be counterbalanced by new and potentially serious neurotoxicity. For this reason, attempts should be made to synthesize new analos which lack this neurotoxicity, but which preserve both the increased lipid solubility and detreased marrow toxicity.