L10 Constitutional Polymorphisms and Anti-Cancer Drugs

• Constitutional polymorphisms are present since fertilization in every cell, show less tolerance for variation, and contribute to disease development and response to chemotherapy.
• Examples of constitutional polymorphisms include Trisomy 13 (Patau syndrome), Trisomy 18 (Edward syndrome), and Trisomy 21 (Down syndrome).
• Somatic mutations are acquired with age and present in individual cells, show more tolerance for variation, and contribute to disease development and response to chemotherapy.
• Somatic mutations target chemotherapy for cancer.

Anti-Cancer Drugs

• Older cancer drugs target highly proliferating cells and DNA synthesis, are not tailored to specific mutations, but patient response is affected by genetics.
• Newer cancer drugs target cancer cell-specific features like genomic translocations (BRC-ABL), overexpressing oncogenes (EGFR-TKIs, B-RAF), and phenotypic characteristics (surface protein expression).
• Cancer cell-specific features are the basis of the selectivity of newer drugs and constitutional/somatic mutations modify their effect.

Dihydropyrimidine Dehydrogenase (DPYD)

• DPYD metabolizes 5-fluorouracil and has been used to treat colorectal, stomach, breast, and pancreatic cancer for 40 years.
• As a pyrimidine analogue, 5-fluorouracil incorporates into DNA and RNA causing cell cycle arrest and apoptosis, as well as inhibits thymidine synthase.
• Side-effects of 5-fluorouracil include diarrhoea, cardiac toxicity (15-30% of patients), and myelosuppression.
• Genetic variants like 2846 A>T (Asp949Val) and 1236 G>A (HAPB3) reduce DPYD activity by 75% in heterozygotes and 50% in homozygotes.
• DYPD2A (splicing defect) and 1679 T>G13 are null alleles, resulting in 50% activity in heterozygotes and no activity in homozygotes.
• Reduced DPYD activity in wild-type contributes to 35% of the general population.
• DPYD intermediate metabolizers have decreased activity at 30-70% of normal, requiring a 50% reduction in starting dose.
• 0.2% of the general population has null DPYD activity, varying across different populations.
• DPYD poor metabolizers (complete DPD deficiency) have increased risk for severe and fatal drug toxicity.
• The FDA recommends that no fluorouracil dose has proven safe in individuals with absent DPD activity.

Irinotecan

• Irinotecan is metabolized by UGT1A1 and used to treat colorectal and lung cancer.
• Carboxylase metabolizes irinotecan to its active form (SN-38), which is inactivated by conjugation of UGT1A1 and UGT1A7 to SN-38 glucuronide.
• Anticancer activity of SN-38 is mediated via inhibition of topoisomerase.
• Side effects of irinotecan include diarrhoea and neutropenia (15-30% of patients).
• Gilbert syndrome is caused by reduced or absent UGT activity by UGT1A1*28, leading to hyperbilirubinemia and jaundice.
• Hyperbilirubinemia describes high bilirubin levels in the blood.
• Jaundice is the yellowing of the skin or whites of the eyes.
• Bilirubin is a by-product of erythrocyte turnover.
• The UGT1A1*28 allele is common in Europeans, characterized by an additional TA repeat in the gene promoter affecting transcription and protein expression.
• 8-78% of the general population are poor metabolizers, depending on ethnic background.
• UGT1A1 genetic variants are linked to irinotecan toxicity, specifically haematological toxicity (neutropenia) in colorectal cancer patients.
• Genotyping of UGT1A1*28 is recommended in USA by the FDA before irinotecan administration.

Cytarabine

• Cytarabine is a backbone therapy for acute myeloid leukaemia.
• Death related toxicity events during consolidation chemotherapy have been displayed by UGT1A1*28 genotype (Johana et al, 2020).
  • 14% homozygotes
  • 1.9% heterozygotes
  • 0.7% homozygote wild type
• TMPT*3A has 2 changes: 480G>A and 719A>G.
• TMPT*3B has a G>A change in 460bp.
• TMPT*3C has a A>G change in 719bp.
• TMPT*4 has a 626-1G>A change.
• Thiopurine S-methyltransferase (TMPT) activity is inversely related to thioguanine nucleotides (TGNs) concentration in patient cells.
• Standard doses of TMPT are likely to develop hematopoietic bone marrow toxicity in heterozygote/deficient patients.
• Dose adjustment is warranted, based on TMPT genotype/phenotype.

Cancer Chemotherapy

• Cancer chemotherapy can be based on tumour cell genotype or phenotype.
• Novel drugs (small molecules and antibodies) targeting tumours can have predicted responses based on tumour genotype or phenotype.
• These agents are targeted specifically to tumour cells, resulting in little or no effect on non-tumour cells.
• Tyrosine kinase inhibitors for ABL-activated cancers and monoclonal antibody therapy targeting CD33-positive leukaemia are examples.

Chronic Myelogenous Leukemia (CML)

• CML is defined by the Philadelphia chromosome, t(9;22), which encodes a fusion oncoprotein with constitutive ABL kinase activity, driving leukemogenesis.
• Mutations of the BCR-ABL kinase domain were found in over 90% of patients with CML who relapsed after an initial response to imatinib.
• Thr315 (T315) is the gatekeeper residue within the ATP-binding pocket.
• Resistance is mediated by mutation at residue 315 (Thr315Ile), preventing hydrogen binding between imatinib and the 315 residue, critical for drug binding.
• Rs12459419 predicts outcome after Gemtuzumab in AML patients.
• 545 adults AML patients and all had a nucleotide mutation and all had intermediate risk cytogenetics.

Herceptin

• Trastuzumab (Herceptin) is a monoclonal antibody to HER2
• HER2 is a tyrosine kinase over-expressed in 20% of breast cancers, driving proliferation and conferring an anti-apoptotic phenotype.
• Over expression is mediated by gene amplification

B-RAF

• B-RAF is a serine-threonine kinase (RAS/RAF/MEK/ERK signalling pathway) activated in human cancer.
• Mutations can arise from the MAP-kinase pathway and are present in:
  • 50-60% melanomas
  • 30-70% thyroid cancers
  • 30% ovarian cancers
  • 10% colorectal cancers
• The most common mutation is BRAF(V600E), resulting in constitutive activation and upregulated cell proliferation.
• Vemurafenib is a small molecule inhibitor of the V600E variant.

Future of Pharmacogenetics in Cancer Therapy

• Different therapies exist to treat cancer including small molecule inhibitors and monoclonal antibody therapies.
• New drugs are based on individual variability which can be:
  • Genetic (tumour and germline)
  • Gene expression (RNA) profiling of tumours
  • Protein expression profiling of tumours
• This should enable individualised therapy by more effective use of existing agents and the development of new agents to target over-expressed genes.