pHLIP® is a platform technology of pH-sensitive peptides: pH (Low) Insertion Peptides that exploit pH differences between healthy and diseased cells as a biomarker for targeting imaging agents and delivering targeted therapy to cells in acidic diseased tissues.
pHLIP® peptides 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.
When the pHLIP® peptide (blue) encounters healthy tissue where the extracellular pH is around pH 7.4, the protonatable residues of pHLIP® peptide (red circles) remain deprotonated and negatively charged, and the peptide is located at or near the hydrophilic outside surface of the cellular membrane. Weakly bound to the membrane, pHLIP® is washed from the membrane via normal perfusion.
When pHLIP® peptide encounters acidic, diseased tissue, it senses the low extracellular pH at cell surfaces (i.e., the concentration of protons (cyan circles) at the surface of the cellular membrane is high), and the protonatable residues and negatively charged C-terminal carboxyl group of pHLIP® peptide have no net charge (green 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® to spontaneously 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 carboxyl group are then exposed to the normal intracellular pH of the cell, they are deprotonated, again becoming negatively charged, and help to anchor pHLIP® peptide in the membrane.
Delivery by pHLIP®
pHLIP® peptides can be used to target and tether cargo molecules to the surfaces of cells in low pH environments of acidic diseased tissues.
The cargo can be an optical marker, a PET, a SPECT, or a MR imaging agent, or an antigen or a protein delivered to induce certain cellular or immune responses.
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 pHLIP® peptide, typically via a link that is unstable inside the cell. Thus, 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 toxins, metabolic activators/inhibitors, 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.