The Journey of Pembrolizumab
Unleashing the Immune System
Pembrolizumab, a humanized anti-PD-1 monoclonal antibody, is infused intravenously to circulate at high concentrations and bind the PD-1 checkpoint receptor on exhausted T cells, blocking its interaction with PD-L1 and PD-L2 on tumor cells — restoring cytotoxic T cell activity and enabling immune-mediated destruction of cancers that had learned to hide from the immune system.
Absorpsi
Pembrolizumab is administered as an intravenous infusion (200 mg
every 3 weeks, or 400 mg every 6 weeks) because it is a large humanized IgG4 monoclonal antibody
(~149 kDa) that cannot survive GI proteolysis and lacks transmucosal absorption. IV infusion
provides 100% bioavailability with immediate peak concentrations. The drug is delivered over
30 minutes. Compared to subcutaneous antibody administration (used for some other monoclonals),
IV infusion avoids injection site reactions and lymphatic absorption delay, achieving immediate
target exposure. No dose adjustment is required for body weight in the approved fixed-dose
regimen — population pharmacokinetic analysis supports fixed dosing as pharmacokinetically
equivalent to weight-based dosing for the clinically approved dose range.
Distribusi
Like adalimumab and other IgG antibodies, pembrolizumab has
a limited volume of distribution (Vd ~6.7 L), consistent with predominant distribution in
plasma and extravascular fluid. The IgG4 subclass was deliberately chosen because it is less
capable of complement activation (CDC) and has reduced ADCC activity compared to IgG1 —
important for an antibody designed to block a receptor rather than kill cells. Pembrolizumab
distributes to tumor-infiltrating lymphocytes within tumors through enhanced permeability
(tumor neovasculature is leaky). FcRn-mediated recycling prolongs its half-life to approximately
27 days. Concentrations in malignant pleural effusion and ascites are substantially lower than
plasma, potentially limiting efficacy in these anatomical compartments.
Mekanisme Kerja
PD-1 (programmed cell death protein 1, CD279) is a transmembrane
receptor expressed on activated T cells, B cells, and NK cells that functions as an immune
checkpoint. After initial T cell activation and clonal expansion, PD-1 upregulation dampens
the immune response to prevent autoimmunity — a normal physiological brake. Tumors exploit
this checkpoint by expressing PD-L1 (CD274, B7-H1) on their surface (often induced by
IFN-gamma from infiltrating lymphocytes) and on tumor-associated macrophages. PD-L1 binding
to PD-1 recruits SHP-1/SHP-2 phosphatases to the PD-1 ITSM domain, dephosphorylating TCR
proximal signaling intermediates (ZAP70, CD3-zeta, Lck), reducing IL-2 production, proliferation,
and cytotoxic T cell effector function — T cell exhaustion. Pembrolizumab binds PD-1 with
high affinity (Kd ~27 pM) via a unique epitope that prevents both PD-L1 and PD-L2 binding,
reverting T cell exhaustion and restoring anti-tumor cytotoxicity.
Metabolisme
As a protein, pembrolizumab is catabolized by the ubiquitous
proteolytic machinery for endogenous IgG. No cytochrome P450 enzymes are involved. The drug
does not interact pharmacokinetically with chemotherapeutic agents, small-molecule TKIs, or
other drugs through conventional metabolic pathways. The primary pharmacokinetic concern is
target-mediated drug disposition (TMDD): at very low concentrations, PD-1 receptor-mediated
internalization increases clearance; as concentrations exceed receptor saturation, clearance
becomes primarily linear. Tumor burden and PD-1 expression level influence individual
clearance variability. Anti-drug antibodies (ADAs) occur in approximately 3-4% of patients
and are of uncertain clinical significance in most cases.
Ekskresi
Pembrolizumab is eliminated entirely by protein catabolism;
no intact antibody appears in urine or feces. The terminal half-life is approximately 27 days.
Clearance is approximately 0.22 L/day and increases with higher tumor burden (more PD-1-
expressing TILs) and decreases as treatment response occurs. The 3-week or 6-week dosing
interval is supported by population PK modeling ensuring trough concentrations well above
the target effective concentration (>1 µg/mL). Renal and hepatic impairment do not require
dose adjustment because elimination is independent of these organs. Age, gender, and mild-to-
moderate organ impairment do not meaningfully affect pembrolizumab pharmacokinetics.
Signifikansi Klinis
Pembrolizumab (Keytruda) is the FDA-approved antibody for the
broadest range of solid tumors, including melanoma, NSCLC, HNSCC, MSI-H/dMMR solid tumors
(tumor-agnostic), gastric cancer, cervical cancer, urothelial cancer, endometrial cancer, TMB-H
cancers, and others. PD-L1 IHC expression score (TPS or CPS) predicts response in several
tumor types. Immune-related adverse events (irAEs) — pneumonitis, colitis, hepatitis,
endocrinopathies (especially thyroiditis and hypophysitis), nephritis, and myocarditis —
result from unleashing T cells against self-tissues, sometimes requiring high-dose
corticosteroids or permanent discontinuation. Combination with chemotherapy, other checkpoint
inhibitors, or targeted therapies is standard in many indications.