6-Thioguanine (Thioguanine; 2-Amino-6-purinethiol) is an anti-leukemia and immunosuppressant agent, acts as an inhibitor of SARS and MERS coronavirus papin-like proteases (PLpros) and also potently inhibits USP2 activity, with IC50s of 25 μM and 40 μM for Plpros and recombinant human USP2, respectively[1]. 6-Thioguanine (Thioguanine) impacts the methylation of cytosine residues by purified DNA methyltransferases including human DNMT1 and bacterial HpaII methylase. 6-Thioguanine (Thioguanine) (1 or 3 μM) lowers global cytosine methylation in Jurkat T cells and cytosine methylation in human cells at 3 μM[2]. 6-Thioguanine (Thioguanine) (18.75, 37.50, or 75.00 μM) significantly impacts cell viability, but with no effect on LDH or ALT activity[3].

Thioguanine is as efficient as a PARP inhibitor in selectively killing BRCA2-defective tumors in a xenograft model. 6-Thioguanine efficiently kills such BRCA1-defective PARP inhibitor-resistant tumors. 6-Thioguanine could kill cells and tumors that have gained resistance to PARP inhibitors or cisplatin through genetic reversion of the BRCA2 gene

Absorption, Distribution and Excretion
Absorption of an oral dose is incomplete and variable, averaging approximately 30% of the administered dose (range: 14% to 46%)
Incompletely and variably (about 30%) absorbed from the gastrointestinal tract.
THIOGUANINE IS INCOMPLETELY ABSORBED WHEN GIVEN ORALLY, AVERAGING ABOUT 30% OF AN ADMIN DOSE. ... THE ELIMINATION HALF-LIFE OF THE PARENT DRUG IS 1.5 HR, BUT PEAK PLASMA LEVELS OF METABOLITES ARE REACHED IN 6-8 HR. BETWEEN 24% & 46% IS EXCRETED IN THE URINE AS METABOLITES WITHIN 24 HR. ... THIS DRUG IS CLEARED RAPIDLY FROM PLASMA AFTER IV ADMINISTRATION; MORE THAN 80% EXCRETED WITHIN 24 HR. ALTHOUGH THIOGUANINE.../SRP: HAS LIMITED ACCESS ACROSS/ THE BLOOD-BRAIN BARRIER IN ANIMALS AFTER LARGE DOSES, VERY LITTLE ENTERS THE CEREBROSPINAL FLUID OF HUMANS AFTER THE USUAL CLINICAL DOSES ARE EMPLOYED.
THE METABOLISM AND PHARMACOKINETICS OF 6-THIOGUANINE WERE STUDIED IN DOGS AFTER IV ADMIN OF 5 MG/KG (35)S-THIOGUANINE (TG). THIOGUANINE WAS RAPIDLY & EXTENSIVELY DEGRADED. METABOLITES WERE NOT FOUND IN THE CEREBROSPINAL FLUID IN SIGNIFICANT CONCENTRATIONS.

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Metabolism / Metabolites
Hepatic. First converted to 6-thioguanilyic acid (TGMP). TGMP is further converted to the di- and tri-phosphates, thioguanosine diphosphate (TGDP) and thioguanosine triphosphate (TGTP) by the same enzymes that metabolize guanine nucleotides.
... WHEN THIOGUANINE IS ADMIN TO MAN, THE S-METHYLATION PRODUCT, 2-AMINO-6-METHYLTHIOPURINE, RATHER THAN FREE THIOGUANINE APPEARS IN URINE; INORGANIC SULFATE IS ALSO A MAJOR URINARY METABOLITE. LESSER AMT OF 6-THIOURIC ACID ARE FORMED, SUGGESTING THAT DEAMINATION CATALYZED BY THE ENZYME GUANASE DOES NOT HAVE A MAJOR ROLE IN THE METABOLIC INACTIVATION OF THIOGUANINE.
THE PHARMACOKINETICS OF RADIOLABELED 6-THIOGUANINE (TG) WERE COMPARED WITH THAT OF BETA-2'-DEOXYTHIOGUANOSINE (BETA-TGDR) AFTER IV ADMIN. URINARY EXCRETION OF THE RADIOLABEL WAS 75% OF THE DOSE 24 HR AFTER ADMIN. BOTH THIOPURINES WERE RAPIDLY & EXTENSIVELY DEGRADED & EXCRETED AS 6-THIOXANTHINE, INORGANIC SULFATE, S-METHYL-6-THIOXANTHINE, & 6-THIOURIC ACID IN ADDITION TO OTHER PRODUCTS. SMALL AMOUNTS OF UNCHANGED DRUG WERE ALSO EXCRETED. STUDIES SUGGEST THAT BETA-TGDR IS A LATENT FORM OF TG. SINCE RESISTANCE TO ANTILEUKEMIC AGENT 6-THIOGUANINE INEVITABLY DEVELOPS IN ANIMAL TUMORS, THIS NEW AGENT BETA-TGDR IS OF POTENTIAL CLINICAL USE.
Hepatic. First converted to 6-thioguanilyic acid (TGMP). TGMP is further converted to the di- and tri-phosphates, thioguanosine diphosphate (TGDP) and thioguanosine triphosphate (TGTP) by the same enzymes that metabolize guanine nucleotides.
Half Life: When the compound was given in singles doses of 65 to 300 mg/m^2, the median plasma half-disappearance time was 80 minutes (range 25-240 minutes)

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Biological Half-Life
When the compound was given in singles doses of 65 to 300 mg/m^2, the median plasma half-disappearance time was 80 minutes (range 25-240 minutes)

Toxicity Summary
Thioguanine competes with hypoxanthine and guanine for the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) and is itself converted to 6-thioguanilyic acid (TGMP), which reaches high intracellular concentrations at therapeutic doses. TGMP interferes with the synthesis of guanine nucleotides by its inhibition of purine biosynthesis by pseudofeedback inhibition of glutamine-5-phosphoribosylpyrophosphate amidotransferase, the first enzyme unique to the de novo pathway of purine ribonucleotide synthesis. TGMP also inhibits the conversion of inosinic acid (IMP) to xanthylic acid (XMP) by competition for the enzyme IMP dehydrogenase. Thioguanine nucleotides are incorporated into both the DNA and the RNA by phosphodiester linkages, and some studies have shown that incorporation of such false bases contributes to the cytotoxicity of thioguanine. Its tumor inhibitory properties may be due to one or more of its effects on feedback inhibition of de novo purine synthesis; inhibition of purine nucleotide interconversions; or incorporation into the DNA and RNA. The overall result of its action is a sequential blockade of the utilization and synthesis of the purine nucleotides.

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Toxicity Data
Rat (ip): LD50 300 mg/kg
Oral, mouse: LD₅₀ = 160 mg/kg.

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Interactions
INDUCTION OF 6-THIOGUANINE RESISTANCE WAS STUDIED IN HUMAN CELLS TREATED WITH THE DIRECT-ACTING CARCINOGEN N-ACETOXY-2-ACETYLAMINOFLUORENE. AT 2.5-7.5 MUMOL INDUCTION OF RESISTANT CLONES WAS LINEAR & FOLLOWED 1-HIT KINETICS, WHILE AT 10 MUMOL THE YIELD OF RESISTANT CLONES WAS HIGHER & APPEARED TO RESULT FROM COMBINATION OF 1-HIT & 2-HIT KINETICS.
6-MERCAPTOPURINE & 6-THIOGUANINE SYNERGISTICALLY INHIBITED MILK XANTHINE OXIDASE.
PRETREATMENT OF L1210 LEUKEMIA CELLS WITH METHOTREXATE ENHANCED CYTOTOXICITY OF 6-THIOGUANINE. METHOTREXATE PRETREATMENT & ENHANCEMENT OF 6-THIOGUANINE CYTOTOXICITY FOLLOWING METHOTREXATE EXPOSURE IS NOT ASSOC WITH 6-THIOGUANINE INCORPORATION INTO DNA, BUT RATHER WITH INCORPORATION OF 6-THIOGUANINE INTO RNA. THIS DRUG SEQUENCE MAY BE BENEFICIAL IN CLINICAL TREATMENT OF LEUKEMIA.

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Non-Human Toxicity Values
LD50 Rat ip 350 mg/kg
LD40 Mouse ip 50 mg/kg

[1]. 6-Thioguanine is a noncompetitive and slow binding inhibitor of human deubiquitinating protease USP2. Sci Rep. 2018 Feb 15;8(1):3102.

[2]. 6-Thioguanine perturbs cytosine methylation at the CpG dinucleotide site by DNA methyltransferases in vitro and acts as a DNA demethylating agent in vivo. Biochemistry. 2009 Mar 17;48(10):2290-9.

[3]. Effects of azathioprine, 6-mercaptopurine, and 6-thioguanine on canine primary hepatocytes. Am J Vet Res. 2015 Jul;76(7):649-55.

Therapeutic Uses
Antimetabolites, Antineoplastic
CLINICALLY, THIOGUANINE HAS BEEN USED IN THE TREATMENT OF ACUTE LEUKEMIA AND, IN COMBINATION WITH CYTARABINE, IS ONE OF THE MOST EFFECTIVE AGENTS FOR INDUCTION OF REMISSIONS IN ACUTE GRANULOCYTIC LEUKEMIA; IT HAS NOT BEEN USEFUL IN THE TREATMENT OF PATIENTS WITH SOLID TUMORS. THIS CMPD HAS BEEN USED AS AN IMMUNOSUPPRESSIVE AGENT, PARTICULARLY IN PATIENTS WITH NEPHROSIS AND WITH COLLAGEN-VASCULAR DISORDERS. TOXIC MANIFESTATIONS INCLUDE BONE MARROW DEPRESSION AND GI EFFECTS, ALTHOUGH THE LATTER MAY BE LESS PRONOUNCED THAN WITH MERCAPTOPURINE.
SHORT TERM TREATMENT WITH DOXORUBICIN, CYTARABINE, & 6-THIOGUANINE WAS GIVEN TO 90 PATIENTS WITH ACUTE MYELOGENOUS LEUKEMIA. FIFTY PATIENTS RECEIVED HIGH DOSES (REGIMEN 1) & 41 RECEIVED VERY HIGH DOSES (REGIMEN 2). REMISSION RATE WAS SIGNIFICANTLY HIGHER WITH REGIMEN 1 THAN WITH REGIMEN 2. DURATION OF REMISSION WAS, HOWEVER, SIGNIFICANTLY LONGER WITH REGIMEN 2.
IN ADVANCED COLORECTAL ADENOCARCINOMA, TWO DIFFERENT SCHEDULES OF COMBINATION METHYL-CCNU, 6-THIOGUANINE, & 5-FLUOROURACIL EXHIBITED SIMILAR EFFICACIES, WITH A COMBINED COMPLETE & PARTIAL REMISSION RATE OF 17% & A MEDIAN SURVIVAL OF 53+ WK. SIGNIFICANT SYMPTOMATIC BENEFIT WAS SEEN IN 52% OF PATIENTS. TOXICITY WAS PREDOMINATELY HEMOPOIETIC & GI.
For more Therapeutic Uses (Complete) data for THIOGUANINE (14 total), please visit the HSDB record page.

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Drug Warnings
Risk-benefit should be considered when the following medical problems exist: Bone marrow depression; chickenpox, existing or recent (including recent exposure; herpes zoster (risk of severe generalized disease); gout, history of; urate renal stones, history of (risk of hyperuricemia); hepatic function impairment (reduced biotransformation; lower dosage is recommended); infection; renal function impairment (reduced elimination; lower dosage is recommended); or sensitivity to thioguanine. Caution should be used also in patients who have had cytotoxic drug therapy and radiation therapy within 4 to 6 weeks.
Because normal defense mechanisms may be suppressed by thioguanine therapy, concurrent use with alive virus vaccine may potentiate the replication of the vaccine virus, may increase the side/adverse effects of the vaccine virus, and/or may decrease the patient's antibody response to the vaccine; immunization of these patients should be undertaken only with extreme caution after careful review of the patient's hematologic status and only with the knowledge and consent of the physician managing the thioguanine therapy. The interval between discontinuation of medication that cause immunosuppression and restoration of the patient's ability to respond to the vaccine depends on the intensity and type of immunosuppression-causing medication used, the underlying disease, and other factors; estimates vary from 3 months to 1 year. Patients with leukemia in remission should not receive live virus vaccine until at least 3 months after their last chemotherapy. Immunization with oral poliovirus vaccine should also be postponed in persons in close contact with the patient, especially family members.
Because normal defense mechanisms may be suppressed by thioguanine therapy, the patient's antibody response to the vaccine may be decreased. The interval between discontinuation of medications that cause immunosuppression and restoration of the patient's ability to respond to the vaccine depends on the intensity and type of immunosuppression-causing medication used, the underlying disease, and other factors; estimates vary from 3 months to 1 year.
TOXIC MANIFESTATIONS INCL BONE MARROW DEPRESSION & GI EFFECTS ... .
For more Drug Warnings (Complete) data for THIOGUANINE (9 total), please visit the HSDB record page.

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Pharmacodynamics
Thioguanine is an antineoplastic anti-metabolite used in the treatment of several forms of leukemia including acute nonlymphocytic leukemia. Anti-metabolites masquerade as purine or pyrimidine - which become the building blocks of DNA. They prevent these substances becoming incorporated in to DNA during the "S" phase (of the cell cycle), stopping normal development and division. Thioguanine was first synthesized and entered into clinical trial more than 30 years ago. It is a 6-thiopurine analogue of the naturally occurring purine bases hypoxanthine and guanine. Intracellular activation results in incorporation into DNA as a false purine base. An additional cytotoxic effect is related to its incorporation into RNA. Thioguanine is cross-resistant with mercaptopurine. Cytotoxicity is cell cycle phase-specific (S-phase).

C5H5N5S

167.1917

167.026

154-42-7

5580-03-0 (hemihydrate);76078-67-6 (mono-Na salt)

2723601

White to yellow solid powder

2.1±0.1 g/cm3

460.7±37.0 °C at 760 mmHg

≥300 °C(lit.)

232.4±26.5 °C

0.0±1.1 mmHg at 25°C

2.071

-0.99

3

2

0

11

225

0

WYWHKKSPHMUBEB-UHFFFAOYSA-N

InChI=1S/C5H5N5S/c6-5-9-3-2(4(11)10-5)7-1-8-3/h1H,(H4,6,7,8,9,10,11)

2-amino-1H-purine-6(7H)-thione

2-Amino-6-purinethiol; thioguanine; ThioguanineTabloid; Tioguanine. Lanvis; Tioguanin; 6TG; TG. BW 5071; WR1141; X 27.

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : ~10 mg/mL (~59.81 mM)

Solubility in Formulation 1: ≥ 1.67 mg/mL (9.99 mM) (saturation unknown) in 10% DMSO + 40% PEG300 +5% Tween-80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 16.7 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 + to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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 (Please use freshly prepared in vivo formulations for optimal results.)