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Structure and Function

Pembrolizumab, or Keytruda, is an immunoglobulin G4 (IgG4)-kappa humanized monoclonal antibody against the programmed cell death-1 (PD-1) receptor. It is a very compact molecule with an asymmetrical Y-shape. The short compact hinge region inflicts constraints on the molecule that creates the abnormal crystallizable tail region (Fc domain) compared to other immunoglobulin G (IgG) proteins. The Fc domain is glycosylated at both CH2 domains on each chain and one of them is distinctively rotated 120° compared to other similar structures, making the glycan chain more solvent accessible and facing the solvent. IgG4s have a unique function where they form dynamic bispecific antibodies by exchanging half-molecules (one heavy chain/light chain pair) among themselves, called Fab-arm exchange. This makes the molecule particularly unstable and unpredictable as a treatment, but can be conquered by introducing a serine-to-proline mutation at amino acid 228, which prevents Fab-arm exchange and stabilizes the molecule ^[3].

Pembrolizumab/PD-1 Interaction

In order for pembrolizumab to block PD-1, pembrolizumab forms a large, flat paratope (antigen-binding site) that can sustain PD-1’s large epitope (where antibody attaches on antigen). The induced interaction between pembrolizumab and PD-1 gives rise to a surface conformational change on PD-1. The new structure of PD-1 becomes a very shallow, “crescent”-like shape, in contrast to it’s flat conformation when bound to PD-L1 ^[4].

PemFv/PD-1 Interaction

The Fv fragment of pembrolizumab (PemFv) can form a complex with the extracellular domain (ECD) of PD-1. Both PemFv and PD-1ECD contain interchain disulfide bonds. PemFv interacts predominantly in the major groove of PD-1, which is formed on one surface by the CC’FG antiparallel β−sheet and the BC, C’D, and FG loops. There are 15 direct hydrogen bonds between the residues, 15 water-mediated hydrogen bonds, 2 salt bridges, and many hydrophobic interactions. A very large solvent-accessible surface area of 1,137Å2 is buried on PD-1ECD due to the convoluted interaction. There are a total of 26 PD-1ECD residues involved in the interaction with PemFv, with residues in loop C’D (Pro84 to Gly90) and strand C’ (Gln75 to Lys 78) playing a major role. These key components of PD-1 mainly form interactions through salt bridges and hydrogen bonds with CRD-L3, CDR-H1, CDR-H2, CDR-H3 of pembrolizumab. It is beleived that the sugar chains of PD-1 have no phsyical contact with pembrolizumab due to the N-linked glycosylated residues (Asn49, Asn58, Asn74, and Asn116) being located away from the interaface ^[4].

PD-L1/PD-1 Interaction

The complex formed when protein-derived ligand, PD-L1, interacts with the inhibitory receptor, PD-1, suppresses immune responses again autoantigens and helps in peripheral immune tolerance. However, when tumors overexpress PD-L1, the interaction with PD-1 inhibits T-lymphocyte proliferation, release of cytokines, and cytotoxicity, exhausting tumor-specific T-cells. There are a total of 12 PD-1ECD residues that are involved in forming the complex with the N-terminal half of PD-L1ECD (PD-L1ECD-N). Nine hydrogen bonds, 3 water-mediated hydrogen bonds, 2 salt bridges, and numerous hydrophobic interactions make up the PD-1ECD/PD-L1ECD-N interaction.The CC’FG sheet within both proteins is the main interaction point. A hydrophobic surface patch is formed when the PD-1ECD is in complex with PD-L1ECD-N. The PD-1ECD residues involved include Val64, Tyr68, Ile126, Leu128, Ala132 and Ile134. Numerous hydrophilic amino acids that encircle PD-L1ECD-N form salt bridges and hydrogen bonds with Asn66, Tyr68, Gln75, Thr76, Asp77, Lys78, Ala132 and Glu136 of PD-1ECD ^[4].

Mechanism

Pembrolizumab works as a PD-1 pathway inhibitor. As an inhibitor it targets the cell death of PD-1 and blocks the immune checkpoint pathway. PD-1 is expressed on the surface of t-cells. T-cells are main components of the immune response in the body. The main ligands that interact with this receptor are PD-L1 and PD-L2, which are expressed by some tumors and inhibit t-cell function when bound to PD-1 ^[5]. Pembrolizumab has a very high affinity to PD-1, allowing it to block the interaction between PD-1 with PD-L1 and PD-L2. It antagonizes the interaction between PD-1 and its known ligands, re-activating anti-tumor immunity ^[6]. The PD-1/PD-L1 interaction inhibits t-lymphocyte proliferation, releases cytokines and cytotoxicity, and exhausts tumor-specific t-cells. The inhibition of this pathway reverses the exhausted t-cell phenotype and normalizes the anti-tumor response. Pembrolizumab may cause inflammatory side effects ^[7].

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