The Journey of Trastuzumab

Trastuzumab is administered intravenously (IV) as the primary route,
with a loading dose of 8 mg/kg followed by 6 mg/kg every 3 weeks (or 4 mg/kg loading then 2 mg/kg
weekly). A subcutaneous formulation co-formulated with recombinant human hyaluronidase (Herceptin SC)
achieves bioavailability of approximately 77% after subcutaneous injection in the thigh, with peak
concentrations at 3-4 days. IV administration provides 100% bioavailability and immediate systemic
exposure. The choice between IV and SC formulations is based on patient preference and clinical
context. Fixed-dose SC trastuzumab (600 mg every 3 weeks) was approved based on pharmacokinetic
bridging studies demonstrating non-inferior trough concentrations compared to weight-based IV dosing.
Oral delivery is not possible due to proteolytic degradation in the GI tract.

Trastuzumab has a volume of distribution of approximately 2.95 L,
essentially equal to plasma volume, reflecting its large molecular weight (~148 kDa) and confinement
to the vascular compartment and interstitial fluid. The IgG1 Fc region mediates binding to FcRn on
endothelial cells, recycling the antibody from lysosomal degradation and prolonging its half-life.
Trastuzumab distributes to HER2-overexpressing tumors via the enhanced permeability and retention
(EPR) effect and through FcRn transport. CNS penetration is poor, explaining the significant rate
of brain metastases in HER2-positive breast cancer patients despite systemic disease control.
Trastuzumab emtansine (T-DM1) conjugates the antibody to a cytotoxic maytansinoid, exploiting
HER2-directed internalization to deliver the toxin intracellularly after receptor-mediated endocytosis.

HER2 (human epidermal growth factor receptor 2, ERBB2) is a receptor
tyrosine kinase in the ErbB family (EGFR/HER1, HER2, HER3, HER4). HER2 has no known direct ligand
but is the preferred heterodimerization partner for all other ErbB receptors. Overexpression (due to
gene amplification in approximately 15-20% of breast cancers and 10-15% of gastric cancers) drives
constitutive dimerization and downstream signaling through PI3K/AKT/mTOR and RAS/RAF/MEK/ERK
pathways, promoting tumor cell proliferation and survival. Trastuzumab binds domain IV of the HER2
extracellular region, sterically blocking receptor dimerization and preventing PI3K and MAPK
signaling. Additionally, it recruits NK cells and macrophages via Fc-gamma receptor binding,
triggering antibody-dependent cellular cytotoxicity (ADCC) that directly kills HER2-positive tumor
cells. Trastuzumab may also prevent ectodomain shedding of HER2 by inhibiting ADAM10/ADAM17.

As a protein therapeutic, trastuzumab is catabolized by the same
ubiquitous proteolytic machinery as endogenous IgG, with no involvement of CYP450 enzymes. The drug
undergoes target-mediated drug disposition (TMDD) at low concentrations, where HER2 receptor
binding and internalization accelerates clearance proportional to tumor HER2 expression. Anti-drug
antibodies (ADAs) are detected in less than 0.1% of patients — very low immunogenicity attributed to
the humanized structure. Concomitant chemotherapy (paclitaxel, pertuzumab) does not significantly
alter trastuzumab pharmacokinetics. Cardiotoxicity — asymptomatic left ventricular dysfunction or
symptomatic heart failure — is the major adverse effect, attributed to inhibition of HER2 signaling
in cardiomyocytes (HER2 plays a role in cardiac stress response via NRG1/HER4 signaling).

Trastuzumab is eliminated entirely by protein catabolism; no intact
antibody appears in urine. The terminal half-life is approximately 28 days (following the loading
dose regimen). Clearance is approximately 0.225 L/day and varies with tumor burden (TMDD at low
concentrations). Steady-state is achieved by approximately week 16 with the standard regimen.
No dose adjustment is required for renal or hepatic impairment. Body weight-based dosing accounts for
pharmacokinetic variability between patients. Extended adjuvant trastuzumab for 1 year (HERA trial)
significantly reduces recurrence in HER2-positive early breast cancer. After completing adjuvant
treatment, plasma levels become undetectable within approximately 4 months (about 3 half-lives).

Trastuzumab transformed HER2-positive breast cancer from a poor-prognosis
subtype to one with favorable outcomes. In metastatic HER2-positive breast cancer, first-line
trastuzumab plus pertuzumab plus docetaxel (CLEOPATRA trial) significantly prolonged overall survival.
In HER2-positive gastric/gastroesophageal junction cancers, trastuzumab plus chemotherapy improved
survival in the ToGA trial. Cardiotoxicity requires baseline and periodic LVEF monitoring; unlike
anthracycline toxicity, trastuzumab-related cardiac dysfunction is generally reversible on
discontinuation. Pertuzumab (targeting HER2 domain II) and trastuzumab deruxtecan (T-DXd, antibody-
drug conjugate with topoisomerase I inhibitor payload) have extended the HER2-targeting paradigm.