pHLIP®s (pH-Low Insertion Peptides) are a platform technology of pH-sensitive peptides that exploit pH differences between healthy and diseased cells as a biomarker for targeting and delivering therapeutic and imaging agents to cells in acidic diseased tissues.

Molecular Mechanism

pHLIP® peptides sense and target pH at cell surfaces, where the pH is the lowest, thus providing high sensitivity.

pHLIP® peptides exploit folding and insertion across the cell membrane, a cooperative process that gives high specificity.

pHLIP® peptides do not target or accumulate in cells with normal surface pH in healthy tissue.

When the pHLIP® peptide (blue) encounters healthy tissue, where the extracellular pH is around pH 7.4, the protonatable residues (red circles) of pHLIP® peptide remain deprotonated and negatively charged, and the peptide is located at the hydrophilic surface of the cell membrane. Weakly bound to the membrane, pHLIP® is washed away from the membrane via normal perfusion.

When a pHLIP® peptide encounters acidic diseased tissue, pHLIP® peptide senses the low extracellular pH, i.e. the concentration of protons (green circles), at the surface of the cellular membrane, and the protonatable residues and negatively charged C-terminal carboxyl group of pHLIP® peptide became neutral and have no net charge (gray circles). The protonation leads to an increase in the overall hydrophobicity of the pHLIP® peptide, increasing the affinity of the peptide to the hydrophobic core of the cellular membrane and triggering pHLIP® peptide to spontaneously partition into the bilayer, fold into a helix and insert its C-terminus across the membrane, resulting in the formation of a stable transmembrane helix.

When the C-terminal, protonatable residue and any other cytoplasmically exposed carboxyl groups are at the normal intracellular pH of the cell, they are deprotonated, again becoming negatively charged, which helps to anchor the pHLIP® peptide in the membrane.

Delivery by pHLIP® peptides

pHLIP® peptides can be used to target and anchor therapeutic and imaging cargo molecules to the surfaces of cells in acidic diseased tissues.

The cargo can be an antigen or a protein delivered to induce certain cellular or immune responses, or an optical marker, a PET, a SPECT, or a MR imaging agent.

pHLIP® peptides can also be used for the intracellular delivery of payloads, facilitating the translocation of therapeutic cargoes across the membranes of cells with low extracellular pH, such as those cells found in acidic diseased tissue.

These payloads are conjugated to the membrane-inserting end of a pHLIP® peptide, typically via a link that is unstable inside the cell (such as an S-S bond). Thus, the pHLIP® peptide targets a payload to acidic diseased cells and flips the cargo across the plasma membrane, releasing it in the cytoplasm.

Therapeutic cargoes may include metabolic activators/inhibitors, cell-regulation agents, chemotherapeutic agents, or agents to alter gene expression.

Multiple pHLIP® peptides can be used to decorate a single nanoparticle, which can range in size from a few to hundreds of nanometers.

Nanocarriers decorated with pHLIP® peptides are biocompatible, can target acidic diseased tissues, and demonstrate enhanced cellular uptake by acidic cells.

Among the pHLIP® peptide-coated nanoparticles that have been investigated are lipid, polymer, and metal-based nanomaterials.