Optimization of the local inhibition of intestinal adenosine deaminase (ADA) by erythro-9-(2-hydroxy-3-nonyl)adenine: enhanced oral delivery of an ADA-activated prodrug for anti-HIV therapy

Adenosine deaminase inhibition

Adenosine deaminase is a critical enzyme in purine metabolism that regulates intra and extracellular adenosine concentrations by converting it to inosine. Adenosine is an important purine that regulates numerous physiological functions by interacting with its receptors. Adenosine and consequently adenosine deaminase can have pro or anti-inflammatory effects on tissues depending on how much time has passed from the start of the injury. In addition, an increase in adenosine deaminase activity has been reported for various diseases and the significant effect of deaminase inhibition on the clinical course of different diseases has been reported. However, the use of inhibitors is limited to only a few medical indications. Data on the increase of adenosine deaminase activity in different diseases and the impact of its inhibition in various cases have been collected and are discussed in this review. Overall, the evidence shows that many studies have been done to introduce inhibitors, however, in vivo studies have been much less than in vitro, and often have not been expanded for clinical use.

Metabolically Generated Stable Isotope-Labeled Deoxynucleoside Code for Tracing DNA N6-Methyladenine in Human Cells

DNA N⁶-methyl-2'-deoxyadenosine (6mdA) is an epigenetic modification in both eukaryotes and bacteria. Here we exploited stable isotope-labeled deoxynucleoside [¹⁵N₅]-2'-deoxyadenosine ([¹⁵N₅]-dA) as an initiation tracer and for the first time developed a metabolically differential tracing code for monitoring DNA 6mdA in human cells. We demonstrate that the initiation tracer [¹⁵N₅]-dA undergoes a specific and efficient adenine deamination reaction leading to the loss the exocyclic amine ¹⁵N, and further utilizes the purine salvage pathway to generate mainly both [¹⁵N₄]-dA and [¹⁵N₄]-2'-deoxyguanosine ([¹⁵N₄]-dG) in mammalian genomes. However, [¹⁵N₅]-dA is largely retained in the genomes of mycoplasmas, which are often found in cultured cells and experimental animals. Consequently, the methylation of dA generates 6mdA with a consistent coding pattern, with a predominance of [¹⁵N₄]-6mdA. Therefore, mammalian DNA 6mdA can be potentially discriminated from that generated by infecting mycoplasmas. Collectively, we show a promising approach for identification of authentic DNA 6mdA in human cells and determine if the human cells are contaminated with mycoplasmas.

Erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) blocks differentiation and maintains the expression of pluripotency markers in human embryonic stem cells

hESCs (human embryonic stem cells) have enormous potential for use in pharmaceutical development and therapeutics; however, to realize this potential, there is a requirement for simple and reproducible cell culture methods that provide adequate numbers of cells of suitable quality. We have discovered a novel way of blocking the spontaneous differentiation of hESCs in the absence of exogenous cytokines by supplementing feeder-free conditions with EHNA [erythro-9-(2-hydroxy-3-nonyl)adenine], an established inhibitor of ADA (adenosine deaminase) and cyclic nucleotide PDE2 (phosphodiesterase 2). hESCs maintained in feeder-free conditions with EHNA for more than ten passages showed no reduction in hESC-associated markers including NANOG, POU5F1 (POU domain class 5 transcription factor 1, also known as Oct-4) and SSEA4 (stage-specific embryonic antigen 4) compared with cells maintained in feeder-free conditions containing bFGF (basic fibroblast growth factor). Spontaneous differentiation was reversibly suppressed by the addition of EHNA, but, upon removing EHNA, hESC populations underwent efficient spontaneous, multi-lineage and directed differentiation. EHNA also acts as a strong blocker of directed neuronal differentiation. Chemically distinct inhibitors of ADA and PDE2 lacked the capacity of EHNA to suppress hESC differentiation, suggesting that the effect is not driven by inhibition of either ADA or PDE2. Preliminary structure–activity relationship analysis found the differentiation-blocking properties of EHNA to reside in a pharmacophore comprising a close adenine mimetic with an extended hydrophobic substituent in the 8- or 9-position. We conclude that EHNA and simple 9-alkyladenines can block directed neuronal and spontaneous differentiation in the absence of exogenous cytokine addition, and may provide a useful replacement for bFGF in large-scale or cGMP-compliant processes.

Identification and characterization of small-molecule ligands that maintain pluripotency of human embryonic stem cells

hESCs (human embryonic stem cells) offer great potential for pharmaceutical research and development and, potentially, for therapeutic use. However, improvements in cell culture are urgently required to allow the scalable production of large numbers of cells that maintain pluripotency. Supplementing feeder-free conditions with either EHNA [erythro-9-(2-hydroxy-3-nonyl)adenine] or readily synthesized analogues of this compound maintains hESC pluripotency in the absence of exogenous cytokines. When the hESC lines SA121 or SA461 were maintained in feeder-free conditions with EHNA they displayed no reduction in stem-cell-associated markers such as Nanog, Oct4 (octamer-binding protein 4) and SSEA4 (stage-specific embryonic antigen 4) when compared with cells maintained in full feeder-free conditions that included exogenously added bFGF (basic fibroblast growth factor). Spontaneous differentiation was reversibly suppressed by the addition of EHNA, but EHNA did not limit efficient spontaneous or directed differentiation following its removal. We conclude that EHNA or related compounds offers a viable alternative to exogenous cytokine addition in maintaining hESC cultures in a pluripotent state and might be a particularly useful replacement for bFGF for large-scale or GMP (good manufacturing practice)-compliant processes.

5'-O-D-valyl ara A, a potential prodrug for improving oral bioavailability of the antiviral agent vidarabine

In order to improve the oral bioavailability of Adenine 9-beta-D-arabinofuranoside (Vidarabine, also called ara A), an antiviral drug which is active against herpes simplex and varicella zoster viruses and the first agent to be licensed for the treatment of systematic herpes virus infection in man, the corresponding 5'-O-D-valyl ester derivative has been synthesized. Based on their physicochemical properties, 5'-O-valyl ara A has emerged as a potential prodrug candidate to improve the oral bioavailability of vidarabine. We describe in this paper a facile synthesis route for the prodrug and its physicochemical properties.

Transport, metabolism and elimination mechanisms of anti-HIV agents

Currently available anti-HIV drugs can be classified into three categories: nucleoside analogue reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, and protease inhibitors. Knowledge of these anti-HIV drugs in various physiological or pharmacokinetic compartments is essential for design and development of drug delivery systems for the treatment of HIV infection. The input and output of anti-HIV drugs in the biological systems are described by their transport and metabolism/elimination in this review. Transport mechanisms of anti-HIV agents across various biological barriers, i.e., gastrointestinal wall, skin, mucosa, blood cerebrospinal barrier, blood-brain barrier, placenta, and cellular membranes, are discussed. Their fates during and after systemic absorption and their metabolism-related drug interactions are reviewed. Many anti-HIV drugs presently marketed in the US bear some significant drawbacks such as relatively short half-life, low bioavailability, poor penetration into the central nervous system, and undesirable side effects. Efforts have been made to design drug delivery systems for the anti-HIV agents to: (1) reduce the dosing frequency; (2) increase the bioavailability and decrease the degradation/metabolism in the gastrointestinal tract; (3) improve the CNS penetration and inhibit the CNS efflux; and (4) deliver them to target cells selectively with minimal side effects. We hope to stimulate further interests in the area of controlled delivery of anti-HIV agents by providing current status of transport and metabolism/elimination of these agents.

Absorption and intestinal metabolism of purine dideoxynucleosides and an adenosine deaminase-activated prodrug of 2',3'-dideoxyinosine in the mesenteric vein cannulated rat ileum

This study investigates the mechanisms of absorption and the role of intestinally localized purine salvage pathway enzymes on the ileal availabilities of 2',3'-dideoxyinosine (ddI), a substrate for purine nucleoside phosphorylase (PNP); 2'-fluoro-2',3'-dideoxyinosine (F-ddI), a non-PNP substrate; and 6-chloro-2',3'-dideoxypurine (6-Cl-ddP), an adenosine deaminase (ADA) activated prodrug of ddI. The potential for increasing the intestinal availability of 6-Cl-ddP through the use of ADA inhibitors, namely, 2'-deoxycoformycin (DCF) and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), is also explored. Drug permeability coefficients across the intestinal epithelium were determined in in situ perfusions in the mesenteric vein cannulated rat ileum based on both drug appearance in blood (Pblood) and disappearance from the lumen (Plumen) and their paracellular and transcellular components were estimated by comparison to the permeabilities of two paracellular markers, mannitol and urea. Values of Pblood for ddI were determined to be (1.1 +/- 0.3) x 10(-6) cm/s, in close agreement with the value of (1.0 +/- 0.3) x 10(-6) cm/s obtained for F-ddI, a PNP resistant analogue of ddI having virtually the same molecular size and lipophilicity as ddI. This indicates that PNP may not play an important role in the low intestinal absorption of ddI. The Pblood for 6-Cl-ddP, (19 +/- 2) x 10(-6) cm/s, was 4.5-fold lower than Plumen, (84 +/- 12) x 10(-6) cm/s, which means that 77 +/- 6% of 6-Cl-ddP was metabolized during its intestinal transport, thus qualitatively accounting for the low oral bioavailability (7%) of 6-Cl-ddP observed in vivo in rats. Extensive intracellular metabolism of 6-Cl-ddP by ADA was confirmed by the high concentrations of ddI found both in the intestinal lumen and blood during 6-Cl-ddP perfusions and by a rate of ddI appearance in blood which was approximately 10-fold higher than ddI controls. Co-perfusion of the potent, hydrophilic ADA inhibitor DCF (Ki = 0. 001-0.05 nM) with 6-Cl-ddP led to only partial inhibition of intestinal ADA, while complete inhibition was obtained using the less potent but more lipophilic inhibitor EHNA (Ki = 1-20 nM). Hence, EHNA may be used to improve intestinal absorption of 6-Cl-ddP in vivo.