Small-Molecule Inhibitors

Definition:

Low–molecular-weight compounds (usually <500 Da) designed to enter cells easily and modulate intracellular targets (enzymes, receptors, signaling proteins) that drive cancer growth and survival.

Key Features for Pharmacists

• Route: Almost always oral (vs. monoclonal antibodies, which are IV).
• Mechanism: Bind directly to protein targets → block enzymatic activity, signaling pathways, angiogenesis, or apoptosis regulators.
• Onset/Duration: Rapid absorption, often daily dosing; half-lives vary (hrs to days).
• Distribution: Lipophilic → penetrate cell membranes (important for CNS activity in some, e.g., lorlatinib).

Main Classes of Small-Molecule Inhibitors

1. Tyrosine Kinase Inhibitors (TKIs):
   • Target receptor or non-receptor kinases (EGFR, ALK, ROS1, BCR-ABL, VEGFR).
   • Examples: Imatinib, Erlotinib, Osimertinib, Alectinib.
2. Serine/Threonine Kinase Inhibitors:
   • e.g., BRAF inhibitors (dabrafenib), MEK inhibitors (trametinib).
3. Angiogenesis Inhibitors:
   • Target VEGFR signaling (sunitinib, sorafenib, pazopanib).
4. Proteasome Inhibitors:
   • Block protein degradation → apoptosis (bortezomib, carfilzomib).
5. PARP Inhibitors:
   • Inhibit DNA repair in BRCA-mutated cancers (olaparib, niraparib).
6. mTOR Inhibitors:
   • Block PI3K/AKT/mTOR pathway (everolimus, temsirolimus).
7. Cell Cycle/CDK Inhibitors:
   • e.g., palbociclib, ribociclib (CDK4/6 inhibitors).
8. Others:
   • Hedgehog inhibitors (vismodegib).
   • IDH inhibitors (ivosidenib, enasidenib).

Clinical Considerations

• Toxicities (class-specific):
  • TKIs: diarrhea, rash, hepatotoxicity, cytopenias, QT prolongation.
  • VEGFR TKIs: hypertension, hand-foot syndrome, bleeding.
  • BRAF/MEK inhibitors: fever, skin toxicity.
  • PARP inhibitors: anemia, fatigue, myelodysplastic risk.
• Drug Interactions:
  • Many are metabolized by CYP3A4 → caution with strong inducers/inhibitors.
  • Acid-reducing agents can impair absorption of some (e.g., erlotinib, dasatinib).
• Monitoring:
  • Depends on agent: ECG, LFTs, renal function, electrolytes, CBC, lipid profile, BP.

Summary for Practice

• SMIs = oral targeted therapies that work intracellularly.
• They require strict adherence, frequent lab & toxicity monitoring, and drug–drug interaction checks.
• Pharmacists play a key role in patient education (adherence, AE recognition) and medication reconciliation.