Targeted Small-Molecule Cancer Therapies

Principles of targeted therapy

Oncogene addiction. Many cancers depend on one mutated driver: a constitutively active kinase, an anti-apoptotic protein, or a defective repair pathway. Small-molecule inhibitors (-ib) are oral, enter cells, and block intracellular enzymes, usually by competing with ATP, whereas monoclonal antibodies (-mab) bind extracellular targets. Benefit requires the target, so tumors are genotyped first. Tumors escape by mutating the drug-binding site, classically a gatekeeper residue (T315I in BCR-ABL, T790M in EGFR), and next-generation inhibitors are designed around these mutations.

Stem decoder: -tinib (tyrosine kinase), -rafenib (BRAF), -ciclib (CDK4/6), -zomib (proteasome), -parib (PARP), -toclax (BCL-2), -denib (IDH), -degib (Smoothened).

Kinase inhibitors in leukemia, GIST, lung, and breast cancer

BCR-ABL and KIT. Imatinib binds the ATP pocket of BCR-ABL, the constitutively active kinase created by t(9;22) in CML and Ph+ ALL, and of KIT and PDGFRA, whose activating mutations drive GIST (CD117-positive tumors of interstitial cells of Cajal). Dasatinib and nilotinib are more potent second-generation agents. The T315I gatekeeper mutation defeats imatinib, dasatinib, nilotinib, and bosutinib but not ponatinib; sunitinib follows imatinib failure in GIST.

EGFR. Activating EGFR mutations (exon 19 deletion, L858R) drive lung adenocarcinoma, mostly in never-smokers, women, and East Asian patients. Erlotinib and gefitinib work until T790M emerges; osimertinib inhibits T790M, penetrates the CNS, and is now first line. Acneiform rash (often paralleling response), diarrhea, and paronychia are class effects.

HER2. Lapatinib reversibly inhibits the intracellular kinase domains of HER2 and EGFR and is paired with capecitabine or letrozole for HER2-positive breast cancer, including after trastuzumab. It causes diarrhea, hepatotoxicity, and rash; LVEF decline is rare. Neratinib and brain-penetrant tucatinib are related.

ALK and ROS1. An EML4-ALK fusion from an inversion on chromosome 2 drives about 5% of lung adenocarcinomas, typically in younger never-smokers. Crizotinib (ALK, ROS1, MET) causes visual disturbances and hepatotoxicity; alectinib and lorlatinib are more potent and better control brain metastases. ROS1-rearranged tumors also respond to crizotinib.

Signaling pathway inhibitors

BRAF and MEK. BRAF V600E, found in about half of cutaneous melanomas and in hairy cell leukemia, papillary thyroid carcinoma, and Langerhans cell histiocytosis, keeps the RAS-RAF-MEK-ERK pathway on. Vemurafenib or dabrafenib is paired with a MEK inhibitor such as trametinib for deeper, longer responses. Alone, a BRAF inhibitor paradoxically activates MAPK signaling in BRAF-wild-type cells with upstream RAS activity, producing keratoacanthomas and cutaneous squamous cell carcinomas.

BTK. Ibrutinib binds cysteine 481 of Bruton tyrosine kinase covalently, silencing B-cell receptor signaling in CLL, mantle cell lymphoma, and Waldenström macroglobulinemia. It causes atrial fibrillation, bleeding from impaired platelet function, hypertension, and infections; an early lymphocytosis reflects cells leaving nodes, not progression. Acalabrutinib and zanubrutinib are more selective.

JAK. Ruxolitinib inhibits JAK1 and JAK2, downstream of JAK2 V617F signaling in myeloproliferative neoplasms. It shrinks the spleen and relieves symptoms in myelofibrosis and treats hydroxyurea-refractory polycythemia vera and steroid-refractory GVHD; anemia, thrombocytopenia, and zoster are expected.

VEGFR multikinase inhibitors. Sorafenib (hepatocellular, renal, thyroid cancer) and sunitinib (renal cell carcinoma, GIST) block VEGFR, PDGFR, and related kinases to starve tumor vasculature. They cause hypertension, proteinuria, bleeding, poor wound healing, hypothyroidism, sunitinib-related heart failure, and hand-foot skin reaction: hyperkeratotic calluses at pressure points, unlike capecitabine's diffuse erythema.

mTOR. Everolimus and temsirolimus bind FKBP12 and inhibit mTORC1, like sirolimus. They treat renal cell carcinoma, HR+ breast cancer (everolimus with exemestane), pancreatic neuroendocrine tumors, and tuberous sclerosis tumors (subependymal giant cell astrocytoma, angiomyolipoma), causing stomatitis, hyperglycemia, hyperlipidemia, pneumonitis, and infections.

Cell cycle, DNA repair, and protein handling

CDK4/6. Mitogens such as estrogen raise cyclin D, which activates CDK4/6 to phosphorylate Rb and release E2F for S-phase entry. Palbociclib, ribociclib, and abemaciclib keep Rb hypophosphorylated and arrest cells in G1, so tumors need intact Rb. With an aromatase inhibitor or fulvestrant they treat HR+/HER2− metastatic breast cancer; neutropenia dominates (abemaciclib: diarrhea; ribociclib: QT prolongation).

PARP. PARP repairs single-strand breaks. Olaparib, niraparib, rucaparib, and talazoparib block and trap it, so breaks become double-strand breaks at replication forks; normal cells repair these by homologous recombination, but BRCA1/2-deficient cells cannot and die (synthetic lethality). Uses: BRCA-mutated ovarian, breast, prostate, and pancreatic cancers. Toxicities: anemia, fatigue, nausea, rare MDS/AML.

Proteasome. Bortezomib (reversible, boron-containing) and carfilzomib (irreversible) inhibit the 26S proteasome. Myeloma cells packed with immunoglobulin accumulate misfolded protein, triggering the unfolded protein response and apoptosis, and undegraded IκB keeps NF-κB off. Bortezomib causes peripheral neuropathy and zoster reactivation (acyclovir prophylaxis); carfilzomib causes less neuropathy but more cardiotoxicity.

Apoptosis and differentiation targets

BCL-2. Venetoclax, a BH3 mimetic, binds BCL-2 and frees BAX and BAK to permeabilize mitochondria. It treats CLL (with an anti-CD20 antibody) and AML in older adults (with azacitidine or decitabine). Killing is so rapid that tumor lysis syndrome is the key hazard, prevented by weekly dose ramp-up, hydration, uric acid lowering, and frequent labs. Neutropenia is common, and strong CYP3A4 inhibitors raise levels.

IDH1/2. Mutant IDH makes the oncometabolite 2-hydroxyglutarate, which inhibits DNA and histone demethylases and blocks myeloid maturation. Ivosidenib (IDH1) and enasidenib (IDH2) let AML blasts mature, which can cause differentiation syndrome (fever, dyspnea, pulmonary infiltrates, edema, hypotension, renal injury); treat promptly with dexamethasone, as with ATRA or arsenic in APL.

Hedgehog. Nearly all basal cell carcinomas have unchecked hedgehog signaling, usually PTCH1 loss that derepresses Smoothened; germline PTCH1 loss (Gorlin syndrome) causes multiple basal cell carcinomas, jaw keratocysts, and medulloblastoma risk. Vismodegib and sonidegib antagonize Smoothened for locally advanced or metastatic disease, causing muscle spasms, dysgeusia, alopecia, and weight loss.

Hedgehog inhibitors are teratogenic: exclude pregnancy and require contraception.