Presented at the Arthritis Advisory Committee on July 15, 2003 by Naomi Winick, M.D.

TPMT Assessment, 6-MP Dosing and Childhood ALL

Years From Study Entry Estimated Survival Percentage Improved Survival in Childhood ALL by Study Era

6-Mercaptopurine Discovered 1951; introduced into clinical trials in 1952 and FDA approved 1953 First clinically useful drug designed and synthesized specifically to act as an anti- purine Converted to thioguanine nucleotides which are incorporated into DNA

6-Mercaptopurine Component of virtually ALL regimens for more than 40 years Daily oral therapy x 2-3 yrs; limited toxicity Dose intensity of 6-MP correlates with EFS (Relling et al, 1999) Synergistic with Methotrexate

Thiopurine Methyltransferase Variability in TPMT activity (Weinshilboum & Sladek, 1980) –88.6% high enzyme activity; 11.1% intermediate; 0.3% undetectable activity Child with ALL (Evans et al, 1991) with severe myelosuppression; exorbitant RBC TGN and TPMT deficiency

TPMT Activity, 6MP, and Childhood ALL TPMT variation impacts clinical response Assessable by phenotype and genotype Can one titrate well without knowledge of pharmacogenetics? Should TPMT activity be determined prospectively in all children with ALL?

TPMT Activity Should Be Determined Prospectively 3-5 cc’s in a GTT Cost $ Avoid severe myelosuppression Prevent second malignancies Not likely to cause significant distress, regardless of genotype determination

Relling & Dervieux Nature Ca Rev 2001;1: Gene for TPMT has A common variant that Causes low activity, High levels of 6MP Metabolites mut/mut wt/mut wt/wt Inherited differences in Metabolism Inherited differences In drug levels Optimal Dose for Each Child

TPMT Activity Should Not Be Mandated Prospectively 1/300 homozygous deficient; dose similar for heterozygotes COG std/low risk trial-projected enrollment children/4 years – Seven children will be deficient, nationwide over 4 years Large institutions new pts/year; one TPMT deficient patient per decade

SJCRH Total XII: Outcome not worse for pts with TPMT defect despite 6MP dose decreases N=19 N=161 P = Blood 93: , 1999 Complete Remission Experience According to TPMT Phenotype Years TPMT Wild Types TPMT Defectives

ALL Induction Therapy A glucocorticoid (Dex vs Pred) Vincristine Asparaginase Intrathecal therapy An anthracycline for higher risk patients

ALL Therapy Consolidation/CNS Prophylaxis Standard Risk patients –Intrathecal therapy –Vincristine –MP and MTX –Glucocorticoid Higher risk patients –Add cyclophosphamide and cytarabine –Intermittent pulses VCR/asparaginase

ALL Therapy Delayed Intensification -Vincristine - Dexamethasone –Asparaginase –Doxorubicin –Cyclophosphamide –Cytarabine –6-Thioguanine –Intrathecal Therapy

ALL Therapy Maintenance Two to Three years –Nightly oral 6-MP –Weekly oral MTX –Intrathecal MTX every 12 weeks –Vincristine/Dexamethasone pulses First introduction of 6-MP Phase not significantly myelosuppressive; protocols adjust to defined ANC

Pts with > 1 defective TPMT allele- lower risk ALL relapse, higher risk brain tumor

Low TPMT and Risk of 2 nd Cancer Following Thiopurine Therapy Higher risk of radiation-induced brain tumors among children with high TGN or TPMT deficiency (Relling et al, 1999) –Unique combination with delivery of oral 6- MP during the delivery of Gy CRT –3/6 with brain tumors had TPMT deficiency

Low TPMT and Risk of sMDS/tAML Following Thiopurine Therapy Trend towards a higher incidence of Etoposide induced AML with lower TPMT activity (p = 0.16) (Relling et al 1998) Greater risk of sMDS/AML with TPMT activity < 14 U/ml RBC (p = 0.03) in NOPHO ALL-92 (Thomsen et al,1999) Therapy did not include Etoposide; total alkylating dose (cyclo)- 3 gm/m2

6-MP-Related Secondary AML 6-MP is ubiquitous among ALL therapies CCG review (Meadows et al, 1989): one t-AML among 9720 children with ALL treated without etoposide Dana Farber review (Kreissman et al, 1990): 2 of 752 t-AML with anthracycline intensive, CRT containing therapy

Summary The availability of TPMT, TGN assessment improves the care of children with ALL Prospective testing of all children will limit, not eliminate, severe myelosuppression and the risk of second malignancy

Summary Rare pts require dramatic dose mod Children with intermediate activity require std or near std doses to maximize the likelihood of cure Risks of inadvertent delays in therapy and/or over-interpretation of a result must be considered

Dosing based on toxicity and thiopurine pcol results: Pts with serious toxicity, or suspected noncompliance Adjust doses based on thiopurine pcol results Pts with TPMT defect 6MP dose decreased preferentially Other drug doses not modified Pts without TPMT Defect All myelosuppressive Drug doses adjusted Demonstrated noncompliance Pts confronted With results and Repeat testing Pts with no Toxicity and WBC in target: NO CHANGE