Thiopurine metabolites – whole blood 6-thioguanine nucleotides (6TGN) and 6-methylmercaptopurine nucleotides (6-MMPN)

Investigation Name:Thiopurine metabolites – whole blood 6-thioguanine nucleotides (6TGN) and 6-methylmercaptopurine nucleotides (6-MMPN)

Alias or Abbreviation:6TGN/6MMPN

Accreditation:Extension for scope, awaiting accreditation due to relocation.

Intro:

Thiopurine drugs, such as mercaptopurine and azathioprine, are widely used to treat inflammatory and autoimmune disease, leukaemia and to prevent rejection post organ transplant. These drugs are catabolised to inactive metabolites by TPMT, which in effect reduces concentrations of the active metabolite, 6-thioguanine nucleotides (6TGN). Indications for TDM of thiopurine metabolites: Treating patients with low TPMT activity Suspected non-compliance or treatment with suboptimal dose Failure to respond to standard doses of thiopurine drugs Patients with deficient or low TPMT activity shunt 6-mercaptopurine towards increased 6-TGN production. Measurement of 6MMPN helps to distinguish patients who are under-dosed or non-compliant, from those demonstrating resistance to thiopurine drugs i.e. preferentially metabolising thiopurine drugs to inactive 6-MMPN rather than 6-TGN. The immunosuppressive effect of thiopurine drugs is mediated primarily by the cytotoxic metabolite, 6TGN, and incorporation of these false bases into DNA. Accumulation of high levels of 6TGN is also responsible for some side effects of thiopurine drugs, and has been associated with leucopenia. Furthermore, high levels of the inactive metabolite 6MMPN, which is formed via the TPMT pathway, may be associated with hepatotoxicity.

Pathology Laboratory:TPMT Laboratory

Requestable Seperately?No

Units:pmol/8x10⁸ cells

Minimum Sample Volume:0.5 mL

Expected Turnaround Time:7 working days

Test Code:6TGN

Sample Type:

EDTA whole blood

Complex Reference Range:235 – 450 pmol 6TGN/8x10⁸ cells - maximum drug efficacy in inflammatory bowel disease.

>5700 pmol 6MMPN/8x10⁸ cells - associated with increased risk of hepatotoxicity.

Collection Conditions / Other Information:Send unseparated. Please store samples prior to dispatch at 4°C.
Samples older than 5 days at receipt are reported with a comment to highlight the age of the sample.
Samples older than 10 days at receipt will not be analysed.
Please provide current thiopurine drug regime and patient diagnosis on the request form.
Steady state levels are reached between 2-4 weeks after a dose change.
TPMT activity can also be undertaken on this sample but must be requested on the form.

Referred to Another Laboratory?No

Storage Requirements:Do not freeze, store at 4°C prior to dispatch. Thiopurine metabolites are stable for <3 days at room temperature, up to 10 days at 4°C.

Posting Address / Requirements:1st Class at ambient temperature Sandwell Health Campus, Specialist Chemistry, Pathology Department, Lyndon, B71 4HJ

NPEX / PDF Reporting Available:Yes

EQA Scheme:We participate in a bi-monthly interlaboratory sample exchange.
Equalis: 527- TPMT genotype, phenotype and thiopurine metabolites. RCPAQAP: Thiopurines

Further information:Thiopurine metabolites have a half-life of several days and so there is no need to take a sample at any special time. We suggest a sample for therapeutic drug monitoring is timed at 4 weeks from the start of treatment or a change in dose.

Lead Contact Details:Consultant Clinical Scientist, Hayley Sharrord-Cole, Email: hayley.sharrod-cole@nhs.net

Email Address For Chasing Results:rwh-tr.bcpsspecialistchemistryenquiries@nhs.net

Methodology:HPLC.

Site Sample Tested:Sandwell Health Campus

Cost:Please email: bcpspathology.info@nhs.net for further details

Contact Number:Tel: 0121 507 5162