The pharmacology of prodrugs of 5-fluorouracil and 1-beta-D-arabinofuranosylcytosine

Cell-free multi-enzyme synthesis and purification of uridine diphosphate galactose

High costs and low availability of UDP‐galactose hampers the enzymatic synthesis of valuable oligosaccharides such as human milk oligosaccharides. Here, we report the development of a platform for the scalable, biocatalytic synthesis and purification of UDP‐galactose. UDP‐galactose was produced with a titer of 48 mM (27.2 g/L) in a small‐scale batch process (200 μL) within 24 h using 0.02 g_enzyme/g_product. Through in‐situ ATP regeneration, the amount of ATP (0.6 mM) supplemented was around 240‐fold lower than the stoichiometric equivalent required to achieve the final product yield. Chromatographic purification using porous graphic carbon adsorbent yielded UDP‐galactose with a purity of 92 %. The synthesis was transferred to 1 L preparative scale production in a stirred tank bioreactor. To further reduce the synthesis costs here, the supernatant of cell lysates was used bypassing expensive purification of enzymes. Here, 23.4 g/L UDP‐galactose were produced within 23 h with a synthesis yield of 71 % and a biocatalyst load of 0.05 g_{total_protein}/g_product. The costs for substrates per gram of UDP‐galactose synthesized were around 0.26 €/g.

Highly enhanced leukemia therapy and oral bioavailability from a novel amphiphilic prodrug of cytarabine

Highly enhanced leukemia therapy and oral bioavailability are demonstrated for a new amphiphilic prodrug of cytarabine.

In-vitro transdermal penetration of cytarabine and its N4-alkylamide derivatives

OBJECTIVES

The aim of this study was to synthesise and determine the transdermal penetration of cytarabine alkylamide derivatives and assess the correlation of flux with physicochemical properties.

METHODS

The alkylamide derivatives of cytarabine were synthesised by acylation at the N4-amino group by the mixed anhydride method. The in-vitro permeation studies were performed using the Franz diffusion cell methodology. Furthermore, partition coefficients (n-octanol-water) and aqueous solubility of the N4-alkylamide derivatives of cytarabine were determined in order to obtain information about their lipophilicity and hydrophilicity.

KEY FINDINGS

The N4-alkylamides of cytarabine (acetyl, butanoyl, hexanoyl, octanoyl, and decanoyl derivatives) showed decreased hydrophilicity and increased lipophilicity. The log D values of the alkylamides were higher than that of the parent compound and increased linearly as the alkyl chain lengthened. N4-hexanoyl-4-amino-1-[(2R,3S,4R,5R)-3,4-dihydroxy-5-(hydroxymethyl)oxolan-2-yl] pyrimidin-2-one) showed the highest median steady-state flux (J(ss)) of 89.0 nmol/cm(2) per h in the series, which shows a high statistical difference with the parent compound flux value (3.70 nmol/cm(2) per h).

CONCLUSIONS

The prodrug approach appears to be a promising strategy for the enhancement of transdermal penetration of cytarabine.

Synthesis and transdermal permeation of novel N4-methoxypoly(ethylene glycol) carbamates of cytarabine

BACKGROUND

Cytarabine is a deoxycytidine analogue commonly used in the treatment of hematological malignant diseases. Its clinical utility, however, is severely limited by its short plasma half-life because of the catabolic action of nucleoside deaminases.

METHOD

In this study, N(4)-carbamate derivatives of cytarabine (1) were synthesized and evaluated for transdermal penetration because this mode of administration may circumvent its limitations. The synthesis of these compounds was achieved in a two-step process. First, the methoxypoly(ethylene glycol) was activated by p-nitrophenyl chloroformate. Second, the activated intermediates were reacted with cytarabine in the presence of N-hydroxysuccinamide to give the N(4)-methoxypoly(ethylene glycol) carbamate derivatives. The transdermal flux values of the N(4)-carbamates of cytarabine were determined in vitro by Franz diffusion cell methodology. Aqueous solubility and log D (pH 7.4) values were determined and assessed for correlation with transdermal flux values.

RESULTS

The synthesized carbamates, particularly, (9)-(13), showed increased solubility in both aqueous and lipid media. Log D values decreased as the oxyethylene chain lengthened.

CONCLUSION

Although none of the derivatives showed significantly higher transdermal penetration than cytarabine (1), it should be mentioned that the mean for cytarabine N(4)-methoxyethyleneoxycarbamate (8) was 10 times higher and the median was 2 times higher.

Enhanced cellular uptake of Ara-C via a peptidomimetic prodrug, L-valyl-ara-C in Caco-2 cells

This study aimed to investigate the gastrointestinal stability and the cellular uptake characteristics of l-valyl-ara-C, a peptidomimetic prodrug of ara-C (cytarabine). After the synthesis of l-valyl-ara-C via the incorporation of l-valine into the N4-amino group of the cytosine ring in ara-C, the gastrointestinal stability of l-valyl-ara-C was examined using artificial gastric juice and artificial intestinal fluids. The cellular uptake characteristics of l-valyl-ara-C were also examined in Caco-2 cells. The disappearance half-life of l-valyl-ara-C was 2.2 h in artificial gastric juice, while the degradation of l-valyl-ara-C was negligible in artificial intestinal fluid and also in the supernatant above the Caco-2 cell monolayer during the 2-h incubation. The cellular accumulation of l-valyl-ara-C was 5-fold higher than that of ara-C in Caco-2 cells. Furthermore, the cellular uptake of l-valyl-ara-C did not increase proportionally to the increase in drug concentration. The cellular accumulation of l-valyl-ara-C was significantly reduced in the presence of uridine, p-aminohippurate, tetraethylammonium and small dipeptides, while it was not changed in the presence of l-valine and benzoic acid, suggesting that l-valyl-ara-C could interact with multiple uptake transporters, including peptide transporters, organic anion and cation transporters and nucleoside transporters, but might not interact with amino acid transporters. In conclusion, l-valyl-ara-C could be effective to improve the oral absorption of ara-C via the carrier-mediated transport pathway.

Pharmacokinetic characteristics of L-valyl-ara-C and its implication on the oral delivery of ara-C

AIM

To evaluate the pharmacokinetic characteristics of L-valyl-ara-C, a peptidomimetic prodrug of ara-C.

METHODS

After the synthesis of L-valyl-ara-C, the in vitro stability of L-valyl-ara-C was examined in various biological media. Plasma pharmacokinetic profiles of ara-C and L-valyl-ara-C were also evaluated in rats.

RESULTS

The degradation of L-valyl-ara-C was negligible in fresh plasma and also in the presence of plasmin over a 2 h incubation period. Furthermore, L-valyl-ara-C appeared to be stable in the leukemia cell homogenates, and subsequently, it was far less cytotoxic than the parent, ara-C in AML2 and L1210 cells. The chemical hydrolysis of L-valyl-ara-C was rather accelerated in acidic pH. Following an oral administration of L-valyl-ara-C, the appearance of ara-C was observed in plasma although the systemic exposure of the prodrug was much higher than that of ara-C. The bioavailability of ara-C was about 4% via prodrug administration.

CONCLUSION

The amide bond of L-valyl-ara-C was stable against the enzymatic hydrolysis, and the utility of L-valyl-ara-C as an oral delivery system of ara-C appeared to be limited by its low metabolic conversion to ara-C in rats.

Antitumor activity of sugar-modified cytosine nucleosides

Nucleoside analogues which show antimetabolic activity in cells have been successfully used in the treatment of various tumors. Nucleosides such as 1-beta-D-arabinofuranosylcytosine (araC), 6-mercaptopurine, fludarabine and cladribine play an important role in the treatment of leukemias, while gemcitabine, 5-fluorouracil and its prodrugs are used extensively in the treatment of many types of solid tumors. All of these compounds are metabolized similarly to endogenous nucleosides and nucleotides. Active metabolites interfere with the de novo synthesis of nucleosides and nucleotides or inhibit the DNA chain elongation after being incorporated into the DNA strand as terminators. Furthermore, nucleoside antimetabolites incorporated into the DNA strand induce strand-breaks and finally cause apoptosis. Nucleoside antimetabolites target one or more specific enzyme(s). The mode of inhibitory action on the target enzyme is not always similar even among nucleoside antimetabolites which have the same nucleoside base, such as araC and gemcitabine. Although both nucleosides are phosphorylated by deoxycytidine kinase and are also good substrates of cytidine deaminase, only gemcitabine shows antitumor activity against solid tumors. This suggests that differences in the pharmacological activity of these nucleoside antimetabolites may reflect different modes of action on target molecules. The design, in vitro cytotoxicity, in vivo antitumor activity, metabolism and mechanism of action of sugar-modified cytosine nucleosides, such as (2'S)-2'-deoxy-2'-C-methylcytidine (SMDC), 1-(2-deoxy-2-methylene-beta-D-erythro-pentofuranosyl)cytosine (DMDC), 1-(2-C-cyano-2-deoxy-1-beta-D-arabino-pentofuranosyl)cytosine (CNDAC) and 1-(3-C-ethynyl-beta-D-ribo-pentofura-nosyl)cytosine (ECyd), developed by our groups, are discussed here.

Effective drug delivery by PEGylated drug conjugates

The current review presents an update of drug delivery using poly(ethylene glycol) (PEG), that focuses on recent developments in both protein and organic drugs. Certainly the past 10 years has resulted in a renaissance of the field of PEG drug conjugates, initiated by the use of higher molecular weight PEGs (M(w)>20,000), especially 40,000 which is estimated to have a plasma circulating t(1/2) of approximately 10 h in mice. This recent resuscitation of small organic molecule delivery by high molecular weight PEG conjugates was founded on meaningful in vivo testing using established tumor models, and has led to a clinical candidate, PEG-camptothecin (PROTHECAN), an ester based prodrug currently in phase II trials. Additional applications of high molecular weight PEG prodrug strategies to amino containing drugs are presented: similar tripartate systems based on lower M(w) PEG and their use with proteins is expounded on. The modification of a benzyl elimination tripartate prodrug specific for mercaptans is presented, and its successful application to 6-mercaptopurine giving a water soluble formulation is discussed. Recent novel PEG oligonucleotides and immunoconjugates are also covered. Clinical results of FDA approved PEGylated proteins are also presented.

In-vitro stability and cytostatic activity of liposomal formulations of 5-fluoro-2'-deoxyuridine and its diacylated derivatives

The water-soluble antineoplastic agent 5-fluoro-2'-deoxyuridine (FUdR) was encapsulated in the water phase of liposomes of different lipid compositions. The retention of this drug upon storage and during contact with plasma was assessed. It was found that, upon refrigeration, diffusion of FUdR across the liposome bilayer was considerably faster when the drug was encapsulated in fluid-type liposomes (egg PC/PS/CHOL) than in solid-type liposomes (DSPC/DPPG/CHOL). With either composition, leakage of the drug from the liposomes was accelerated upon contact with plasma. To achieve improved liposomal retention of the drug, FUdR was converted to a lipophilic prodrug by esterifying the free hydroxyl groups in the deoxyribose moiety with fatty acids of different chain lengths. Thus FUdR-dipalmitate (C-16) and FUdR-dioctanoate (C-8) were synthesized and incorporated in liposomes. The dipalmitoyl derivative could be incorporated upto 13 mol% in solid-type liposomes but to only 2 mol% in fluid-type liposomes. Freeze-fracture electron microscopy revealed no major differences between control liposomes and those containing the prodrug. FUdR-dipalmitate was found to be firmly associated with the liposomal bilayer in both liposome-types: no exchange of the pro-drug with blood constituents or hydrolysis by serum esterases could be registered when the liposomes were incubated with serum. On the other hand, liposome-incorporated FUdR-dioctanoate was found to be readily extracted from the liposomes by serum components (predominantly albumin) and was found to be degraded rapidly by serum esterase activity. The antitumor activity of FUdR-prodrugs was determined using C26 colon adenocarcinoma cells. This cell line was found to be highly sensitive to FUdR. Liposomal FUdR-dioctanoate inhibited cell growth in the same concentration range as unesterified FUdR. FUdR-dipalmitate, however, was more than two orders of magnitude less potent in inhibiting cell proliferation. Its antiproliferative activity was dependent on the liposome-type used: when incorporated in fluid-type liposomes, antiproliferative activity of FUdR-dipalmitate was several-fold higher than in solid-type liposomes. The difference in antitumor activity between FUdR-dipalmitate and FUdR-dioctanoate and between FUdR-dipalmitate in the fluid- and solid-type liposomes could be explained by differences in the rate of hydrolysis of the prodrugs to FUdR by esterase activity in the tumor cells or in the growth medium.

Biochemical pharmacology and analysis of fluoropyrimidines alone and in combination with modulators

After more than three decades since their introduction, fluoropyrimidines, especially FUra, are still a mainstay in the treatment of various solid malignancies. The antitumor effects of fluoropyrimidines are dependent upon metabolic activation. FdUMP, FUTP and FdUTP were identified as the key cytotoxic metabolites that interfere with the proper function of thymidylate synthase and nucleic acids. The relevance of these metabolites is cell-type specific. Recently, fluorouridine diphospho sugars have been detected, but the precise function of this class of metabolites is currently unknown. In mammalian systems fluoropyrimidines and their natural counterparts share the same metabolic pathways since the substrate properties in enzyme-catalyzed reactions are frequently comparable. Ongoing studies indicate that the metabolism and action of fluoropyrimidines exhibit circadian rhythms, which appear to be due to variations in the activity of metabolizing enzymes. Essential for the expanding knowledge of the pathways and effects of fluoropyrimidines has been the constant improvement of analytical methods. These include ligand binding techniques, numerous dedicated HPLC systems and 19F-NMR. Because the overall response rates achieved with fluoropyrimidines are modest, strategies based on biochemical modulation have been devised to enhance their therapeutic index. Biochemical modulators include a wide range of various compounds with different modes of action. In recently completed clinical trials, combinations of FUra with leucovorin, a precursor for 5,10-methylene tetrahydrofolate, or with levamisole, an anthelminthic with immunomodulatory activity, appeared to be superior to FUra alone. At the preclinical level combinations of fluoropyrimidines with, e.g. interferons or L-histidinol were demonstrated to be interesting candidates for further testing. The future therapeutic utility of fluoropyrimidines will depend on both the improvement of combination regimens currently used in the treatment of cancer patients and the judicious clinical implementation of promising experimental modulation strategies. Moreover, novel fluoropyrimidines with superior pharmacological properties may become important as part of or instead of modulation approaches.