Page 90 - Read Online

Franca et al. Cancer Drug Resist 2019;2:256-70 I http://dx.doi.org/10.20517/cdr.2019.004 Page 257

A B C

Figure 1. Thiopurines chemical structure. A: thioguanine; B: mercaptopurine; C: azathioprine

[1]
of the mainstay maintenance therapy ; instead, the MP prodrug azathioprine (AZA, Figure 1C) is employed
in non-malignant conditions such as inflammatory bowel disease (IBD), including Crohn’s disease (CD) and
[2]
ulcerative colitis (UC) during the maintenance phase of treatment .
Thiopurines are antimetabolites similar in structure to purines: in particular MP is an analogue of
hypoxanthine and TG of guanine. In cells, thiopurines undergo complex anabolic and catabolic processes. The
anabolic pathway produces thionucleotides (TGN), including thioguanosine mono-, di-, tri-phosphate (tGMP,
tGDP, tGTP) and deoxythioguanosine mono-, di-, tri-phosphate (tdGMP, tdGDP, tdGTP), associated with
therapeutic efficacy. The conversion of MP to tdGMP/tGMP involves the consecutive action of the enzymes
of salvage pathway of nucleotides biosynthesis, in particular hypoxanthine phosphoribosyltransferase 1
(HPRT1), inositol monophosphate dehydrogenase (IMPDH) and guanosine monophosphate synthetase
(GMPS), whereas TG is directly converted by HPRT1 in a single step reaction [Figure 2]. The resulting tdGTP/
tGTP thionucleotides antagonize the incorporation of canonical dGTP and GTP into DNA and RNA, thus
impairing DNA and RNA polymerases and subsequently inducing cell-cycle arrest and apoptosis because
[3]
of altered DNA, RNA and protein synthesis . Furthermore, the cytotoxic action of these drugs in lymphoid
cells is implemented by additional mechanisms of action such as the inhibition of the purine de novo synthesis
pathway and tGTP-mediated inhibition of Ras-related C3 botulinum toxin substrate 1 (Rac-1), a GTPase of
[3]
the Rho family . Catabolic pathways of thiopurines are mediated by the enzymes xanthine oxidase (XO)
and thiopurine methyltransferase (TPMT). The extensive first-pass metabolism of the drug by XO in the
liver and intestinal mucosa is responsible for the low bioavailability of oral MP (less than 20%) and generates
the main inactive metabolite, 6-thiouric acid, excreted in the urine . Indeed, it is known that allopurinol, a
[4]
structural isomer of hypoxanthine and a XO inhibitor, influences thiopurine pharmacokinetics and promotes
TGN production . MP and TG are also converted into methylmercaptopurine (MMP) derivatives by TPMT.
[4]
This reaction can occur also in cells different from hepatocytes, since TPMT is ubiquitously expressed.
MMP is not converted to nucleotides, as it is a poor HPRT substrate and has no antileukemic activity . The
[5]
synthesis of MMP therefore is in competition with the anabolic pathway of thiopurines. TPMT catalyses
the S-methylation also of intermediate thionucleotides leading to TGN, producing secondary methylated
nucleotides (MMPN), with potential cytotoxic activity through the inhibition of de novo purine synthesis. The
[6]
balance between TGN and MMPN has been related to thiopurines response and cytotoxicity . In IBD, TGN
8
concentrations between 230 and 450 pmol/8 × 10 red blood cells (RBC) are associated with the therapeutic
index and clinical efficacy, while higher TGN concentrations have been related to myelosuppression and
8
[7]
other severe complications such as infections ; concentrations of MMPN above 5,700 pmol/8 × 10 RBC have
[8]
been associated with hepatotoxicity . To authors’ knowledge, there is no general consensus for the range
of TGN/MMPN plasma concentrations that should be achieved for drug efficacy and toxicity in ALL: some
clinical protocols suggest MP dose reduction with TGN concentration above 1000 pmol/8 × 10 RBC in order
8