Translocation of nucleoside analogs across the plasma membrane in hematologic malignancies

Review of Transporter Substrate, Inhibitor, and Inducer Characteristics of Cladribine

Cladribine is a nucleoside analog that is phosphorylated in its target cells (B- and T-lymphocytes) to its active adenosine triphosphate form (2-chlorodeoxyadenosine triphosphate). Cladribine tablets 10 mg (Mavenclad^®) administered for up to 10 days per year in 2 consecutive years (3.5-mg/kg cumulative dose over 2 years) are used to treat patients with relapsing multiple sclerosis. The ATP-binding cassette, solute carrier, and nucleoside transporter substrate, inhibitor, and inducer characteristics of cladribine are reviewed in this article. Available evidence suggests that the distribution of cladribine across biological membranes is facilitated by a number of uptake and efflux transporters. Among the key ATP-binding cassette efflux transporters, only breast cancer resistance protein has been shown to be an efficient transporter of cladribine, while P-glycoprotein does not transport cladribine well. Intestinal absorption, distribution throughout the body, and intracellular uptake of cladribine appear to be exclusively mediated by equilibrative and concentrative nucleoside transporters, specifically by ENT1, ENT2, ENT4, CNT2 (low affinity), and CNT3. Renal excretion of cladribine appears to be most likely driven by breast cancer resistance protein, ENT1, and P-glycoprotein. The latter may play a role despite its poor cladribine transport efficiency in view of the renal abundance of P-glycoprotein. There is no evidence that solute carrier uptake transporters such as organic anion transporting polypeptides, organic anion transporters, and organic cation transporters are involved in the transport of cladribine. Available in vitro studies examining the inhibitor characteristics of cladribine for a total of 13 major ATP-binding cassette, solute carrier, and CNT transporters indicate that in vivo inhibition of any of these transporters by cladribine is unlikely.

Cellular Uptake and Intracellular Phosphorylation of GS-441524: Implications for Its Effectiveness against COVID-19

GS-441524 is an adenosine analog and the parent nucleoside of the prodrug remdesivir, which has received emergency approval for treatment of COVID-19. Recently, GS-441524 has been proposed to be effective in the treatment of COVID-19, perhaps even being superior to remdesivir for treatment of this disease. Evaluation of the clinical effectiveness of GS-441524 requires understanding of its uptake and intracellular conversion to GS-441524 triphosphate, the active antiviral substance. We here discuss the potential impact of these pharmacokinetic steps of GS-441524 on the formation of its active antiviral substance and effectiveness for treatment of COVID-19. Available protein expression data suggest that several adenosine transporters are expressed at only low levels in the epithelial cells lining the alveoli in the lungs, i.e., the alveolar cells or pneumocytes from healthy lungs. This may limit uptake of GS-441524. Importantly, cellular uptake of GS-441524 may be reduced during hypoxia and inflammation due to decreased expression of adenosine transporters. Similarly, hypoxia and inflammation may lead to reduced expression of adenosine kinase, which is believed to convert GS-441524 to GS-441524 monophosphate, the perceived rate-limiting step in the intracellular formation of GS-441524 triphosphate. Moreover, increases in extracellular and intracellular levels of adenosine, which may occur during critical illnesses, has the potential to competitively decrease cellular uptake and phosphorylation of GS-441524. Taken together, tissue hypoxia and severe inflammation in COVID-19 may lead to reduced uptake and phosphorylation of GS-441524 with lowered therapeutic effectiveness as a potential outcome. Hypoxia may be particularly critical to the ability of GS-441524 to eliminate SARS-CoV-2 from tissues with low basal expression of adenosine transporters, such as alveolar cells. This knowledge may also be relevant to treatments with other antiviral adenosine analogs and anticancer adenosine analogs as well.

The Clinical Pharmacology of Cladribine Tablets for the Treatment of Relapsing Multiple Sclerosis

Cladribine Tablets (MAVENCLAD^®) are used to treat relapsing multiple sclerosis (MS). The recommended dose is 3.5 mg/kg, consisting of 2 annual courses, each comprising 2 treatment weeks 1 month apart. We reviewed the clinical pharmacology of Cladribine Tablets in patients with MS, including pharmacokinetic and pharmacometric data. Cladribine Tablets are rapidly absorbed, with a median time to reach maximum concentration (T_max) of 0.5 h (range 0.5–1.5 h) in fasted patients. When administered with food, absorption is delayed (median T_max 1.5 h, range 1–3 h), and maximum concentration (C_max) is reduced by 29% (based on geometric mean). Area under the concentration–time curve (AUC) is essentially unchanged. Oral bioavailability of cladribine is approximately 40%, pharmacokinetics are linear and time-independent, and volume of distribution is 480–490 L. Plasma protein binding is 20%, independent of cladribine plasma concentration. Cladribine is rapidly distributed to lymphocytes and retained (either as parent drug or its phosphorylated metabolites), resulting in approximately 30- to 40-fold intracellular accumulation versus extracellular concentrations as early as 1 h after cladribine exposure. Cytochrome P450-mediated biotransformation of cladribine is of minor importance. Cladribine elimination is equally dependent on renal and non-renal routes. In vitro studies indicate that cladribine efflux is minimally P-glycoprotein (P-gp)-related, and clinically relevant interactions with P-gp inhibitors are not expected. Cladribine distribution across membranes is primarily facilitated by equilibrative nucleoside transporter (ENT) 1, concentrative nucleoside transporter (CNT) 3 and breast cancer resistance protein (BCRP), and there is no evidence of any cladribine-related effect on heart rate, atrioventricular conduction or cardiac repolarisation (QTc interval prolongation). Cladribine Tablets are associated with targeted lymphocyte reduction and durable efficacy, with the exposure–effect relationship showing the recommended dose is appropriate in reducing relapse risk.

Inhibitor selectivity of CNTs and ENTs

The concentrative nucleoside transporters (CNT; solute carrier family 28 (SLC28)) and the equilibrative nucleoside transporters (ENT; solute carrier family 29 (SLC29)) are important therapeutic targets but may also mediate toxicity or adverse events. To explore the relative role of the base and the monosaccharide moiety in inhibitor selectivity we selected compounds that either harbor an arabinose moiety or a cytosine moiety, as these groups had several commercially available drug members. The screening data showed that more compounds harboring a cytosine moiety displayed potent interactions with the CNTs than compounds harboring the arabinose moiety. In contrast, ENTs showed a preference for compounds with an arabinose moiety. The correlation between CNT1 and CNT3 was good as five of six compounds displayed IC₅₀ values within the threefold threshold and one displayed a borderline 4-fold difference. For CNT1 and CNT2 as well as for CNT2 and CNT3 only two of six IC₅₀ values correlated and one displayed a borderline 4-fold difference. Interestingly, of the six compounds that potently interacted with both ENT1 and ENT2 only nelarabine displayed selectivity. Our data show differences between inhibitor selectivities of CNTs and ENTs as well as differences within the CNT family members.

New perspectives on the roles of pyrimidines in the central nervous system

Since 1956, when exogenous uridine and cytidine were found to be necessary for the maintenance of perfused rat brain function, the co-existence of de novo synthesis, salvage pathways and removal of pyrimidine bases in the CNS has been a controversial subject. Here, we review studies on metabolites and enzymes of pyrimidine metabolism through more than 60 years. In view of known and newly-described inherited pyrimidine and purine disorders - some with complex clinical profiles of neurological impairments - we underline the necessity to investigate how the different pathways work together in the developing brain and then sustain plasticity, regeneration and neuro-transmission in the adult CNS. Experimentally, early incorporation studies in animal brain slices and homogenates with radio-labelled nucleosides or precursors demonstrated salvage activity or de novo synthesis. Later, the nucleoside transporters and organic anionic transporters underlying uptake of metabolites and anti-pyrimidine drugs in the CNS were identified. Recently, the expression of de novo enzymes in glial cells and neurons was verified using (immuno) histochemical and in-situ-hybridization techniques. Adult brain was shown to take up or produce all pyrimidine (deoxy) ribonucleosides or, after uptake and phosphorolysis of nucleosides, to make use of ribose for different purposes, including energy. More recently, non-canonical pyrimidine bases (5mC, 5hmC) have been found most notably in brain, pointing to considerable postreplicative DNA metabolism, with the need for pyrimidine-specific enzymes. Even more perspectives are emerging, with advances in genome analysis and in the manipulation of expression from the gene.

Relationship between Azathioprine metabolites and therapeutic efficacy in Chinese patients with neuromyelitis optica spectrum disorders

BACKGROUND

Neuromyelitis optica spectrum disorders (NMOSD) are demyelinating autoimmune diseases in the central nervous system (CNS) that are characterized by a high relapse rate and the presence of anti-aquaporin 4 antibodies (AQP4-IgG) in the serum. Azathioprine (AZA) is a first-line immunomodulatory drug that is widely used for the treatment of patients with NMOSD. However, the efficacy and safety of AZA vary in different individuals.

METHOD

Thirty-two patients with NMOSD who regularly took AZA were enrolled in the study at Beijing Tiantan Hospital, Capital Medical University. The efficacy of AZA was evaluated using the expanded disability status scale (EDSS) and the annual relapse rate (ARR). The erythrocyte concentrations of AZA metabolites were detected using an LC-MS/MS method.

RESULTS

The erythrocyte concentrations of 6-thioguanine nucleotides (6-TGNs) and 6-methylmercaptopurine nucleotides (6-MMPNs) were 202.03 ± 63.35 pmol/8*10⁸ RBC and 1618.90 ± 1607.06 pmol/8*10⁸ RBC, respectively. After the patients had received AZA therapy for more than one year, the EDSS score decreased from 5.21 ± 0.24 to 2.57 ± 0.33 (p < 0.0001), and the ARR decreased from 1.41 ± 0.23 to 0.36 ± 0.09 (p < 0.0001). The 6-TGN and 6-MMPN levels were significantly different between the non-relapsed and relapsed groups (p < 0.0001, p = 0.006, respectively). A higher ARR was significantly correlated with higher erythrocyte concentrations of 6-TGNs (p < 0.0001) and 6-MMPNs (p = 0.004).

CONCLUSION

AZA can reduce the EDSS score and ARR in NMOSD patients. Additionally, the efficacy of AZA is significantly related to the erythrocyte concentrations of 6-TGNs and 6-MMPNs. Within the safe upper limits, a higher concentration of 6-TGNs is associated with better efficacy of AZA.

TRIAL REGISTRATION NUMBER

ISRCTN16551495 , retrospectively registered on May 22, 2017.

FLT3 is implicated in cytarabine transport by human equilibrative nucleoside transporter 1 in pediatric acute leukemia

FLT3 abnormalities are negative prognostic markers in acute leukemia. Infant leukemias are a subgroup with frequent MLL (KMT2A) rearrangements, FLT3 overexpression and high sensitivity to cytarabine, but dismal prognosis. Cytarabine is transported into cells by Human Equilibrative Nucleoside Transporter-1 (hENT1, SLC29A1), but the mechanisms that regulate hENT1 in acute leukemia have been scarcely studied.We explored the expression and functional link between FLT3 and main cytarabine transporters in 50 pediatric patients diagnosed with acute lymphoblastic leukemia and MLL rearrangement (ALL-MLL+) and other subtypes of leukemia, and in leukemia cell lines.A significant positive correlation was found between FLT3 and hENT1 expression in patients. Cytarabine uptake into cells was mediated mainly by hENT1, hENT2 and hCNT1. hENT1-mediated uptake of cytarabine was transiently abolished by the FLT3 inhibitor PKC412, and this effect was associated with decreased hENT1 mRNA and protein levels. Noticeably, the cytotoxicity of cytarabine was lower when cells were first exposed to FLT3 inhibitors (PKC412 or AC220), probably due to decreased hENT1 activity, but we observed a higher cytotoxic effect if FLT3 inhibitors were administered after cytarabine.FLT3 regulates hENT1 activity and thereby affects cytarabine cytotoxicity. The sequence of administration of cytarabine and FLT3 inhibitors is important to maintain their efficacy.

Inosine strongly enhances proliferation of human C32 melanoma cells through PLC-PKC-MEK1/2-ERK1/2 and PI3K pathways

Malignant melanoma is the most deadly type of skin cancer. The lack of effective pharmacological approaches for this tumour can be related to the incomplete understanding of the pathophysiological mechanisms involved in melanoma cell proliferation. Adenosine has growth-promoting and growth inhibitory effects on tumour cells. We aimed to investigate effects of adenosine and its metabolic product, inosine, on human C32 melanoma cells and the signalling pathways involved. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) reduction and bromodeoxyuridine (BrdU) proliferation assays were used to evaluate adenosine, adenosine deaminase and inosine effects, in the absence or presence of adenosine receptor (AR), A3 AR and P2Y1 R antagonists and PLC, PKC, MEK1/2 and PI3K inhibitors. ERK1/2 levels were determined using an ELISA kit. Adenosine and inosine levels were quantified using an enzyme-coupled assay. Adenosine caused cell proliferation through AR activation. Adenosine deaminase increased inosine levels (nanomolar concentrations) on the extracellular space, in a time-dependent manner, inducing proliferation through A3 AR activation. Micromolar concentrations of inosine enhanced proliferation through A3 AR activation, causing an increase in ERK1/2 levels, and P2Y1 R activation via ENT-dependent mechanisms. We propose the simultaneous activation of PLC-PKC-MEK1/2-ERK1/2 and PI3K pathways as the main mechanism responsible for the proliferative effect elicited by inosine and its significant role in melanoma cancer progression.

Purine analog-like properties of bendamustine underlie rapid activation of DNA damage response and synergistic effects with pyrimidine analogues in lymphoid malignancies

Bendamustine has shown considerable clinical activity against indolent lymphoid malignancies as a single agent or in combination with rituximab, but combination with additional anti-cancer drugs may be required for refractory and/or relapsed cases as well as other intractable tumors. In this study, we attempted to determine suitable anti-cancer drugs to be combined with bendamustine for the treatment of mantle cell lymphoma, diffuse large B-cell lymphoma, aggressive lymphomas and multiple myeloma, all of which are relatively resistant to this drug, and investigated the mechanisms underlying synergism. Isobologram analysis revealed that bendamustine had synergistic effects with alkylating agents (4-hydroperoxy-cyclophosphamide, chlorambucil and melphalan) and pyrimidine analogues (cytosine arabinoside, gemcitabine and decitabine) in HBL-2, B104, Namalwa and U266 cell lines, which represent the above entities respectively. In cell cycle analysis, bendamustine induced late S-phase arrest, which was enhanced by 4-hydroperoxy-cyclophosphamide, and potentiated early S-phase arrest by cytosine arabinoside (Ara-C), followed by a robust increase in the size of sub-G1 fractions. Bendamustine was able to elicit DNA damage response and subsequent apoptosis faster and with shorter exposure than other alkylating agents due to rapid intracellular incorporation via equilibrative nucleoside transporters (ENTs). Furthermore, bendamustine increased the expression of ENT1 at both mRNA and protein levels and enhanced the uptake of Ara-C and subsequent increase in Ara-C triphosphate (Ara-CTP) in HBL-2 cells to an extent comparable with the purine analog fludarabine. These purine analog-like properties of bendamustine may underlie favorable combinations with other alkylators and pyrimidine analogues. Our findings may provide a theoretical basis for the development of more effective bendamustine-based combination therapies.

Basolateral uptake of nucleosides by Sertoli cells is mediated primarily by equilibrative nucleoside transporter 1

The blood-testis barrier (BTB) prevents the entry of many xenobiotic compounds into seminiferous tubules thereby protecting developing germ cells. Understanding drug transport across the BTB may improve drug delivery into the testis. Members of one class of drug, nucleoside reverse transcriptase inhibitors (NRTIs), do penetrate the BTB, presumably through interaction with physiologic nucleoside transporters. By investigating the mechanism of nucleoside transport, it may be possible to design other drugs to bypass the BTB in a similar manner. We present a novel ex vivo technique to study transport at the BTB that employs isolated, intact seminiferous tubules. Using this system, we found that over 80% of total uptake by seminiferous tubules of the model nucleoside uridine could be inhibited by 100 nM nitrobenzylmercaptopurine riboside (NBMPR, 6-S-[(4-nitrophenyl)methyl]-6-thioinosine), a concentration that selectively inhibits equilibrative nucleoside transporter 1 (ENT1) activity. In primary cultured rat Sertoli cells, 100 nM NBMPR inhibited all transepithelial transport and basolateral uptake of uridine. Immunohistochemical staining showed ENT1 to be located on the basolateral membrane of human and rat Sertoli cells, whereas ENT2 was located on the apical membrane of Sertoli cells. Transepithelial transport of uridine by rat Sertoli cells was partially inhibited by the NRTIs zidovudine, didanosine, and tenofovir disoproxil fumarate, consistent with an interaction between these drugs and ENT transporters. These data indicate that ENT1 is the primary route for basolateral nucleoside uptake into Sertoli cells and a possible mechanism for nucleosides and nucleoside-based drugs to undergo transepithelial transport.