The development of therapeutic peptide-nucleic acid conjugates selective for hypoxic cancer cells

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The epidermal growth factor receptor (EGFR) plays an important role in cancer cell proliferation and is a well-established drug target in the treatment of several cancers, including breast cancer. However, EGFR plays important roles in the control of proliferation and cell fate in non-cancerous cells. It is, therefore, unsurprising that systemic inhibition of EGFR is associated with severe and unpleasant side effects including haemorrhage, cardiac arrhythmia, porphyria, peripheral neuropathy, hepatoxicity, insomnia, nausea, vomiting, diarrhoea, alopecia, and pyrexia. Unlike most proteins, EGFR continues to be expressed in the hypoxic conditions seen in cancer cells, and the Spriggs lab has recently identified the mechanism by which this occurs. We have data to show that a post-transcriptional gene regulation mechanism involving the 5' untranslated region of EGFR mRNA allows EGFR expression under hypoxic conditions. Silencing hypoxic expression of EGFR using nucleic acids complementary to EGFR mRNA would enable selective destruction of cancer cells. However, the development of a viable treatment using this technique requires targeted delivery of the therapeutic nucleic acids. The Mitchell lab has recently developed a novel reaction that enables the modification of peptides and proteins via light-activated chemistry. Our strategy enables the installation of desired functionality into polypeptides via a stable and biogenic carbon-carbon bond. This chemistry is rapid, operationally simple, and selective in the presence of the native chemical functionality of peptides and nucleic acids, making it ideal for the preparation of peptide-nucleic acid (oligonucleotide) conjugates. The aim of this project is to employ visible-light-mediated bioconjugation to prepare peptide-oligonucleotide conjugates to selectively target and silence EGFR expression only in hypoxic cells. Attachment of EGFR-targeting peptides to oligonucleotides will enable selective delivery into cells that express this cell-surface receptor. Although many cancers aberrantly express EGFR, reliance on this method of targeting alone will cause healthy cells to internalise the conjugate. Since our strategy also involves a nucleic acid sequence that binds to EGFR mRNA only present in hypoxic cells, the conjugate will only be therapeutically active in hypoxic cells. This approach will ensure that healthy cells would be spared, and unpleasant side-effects of systemic EGFR inhibition avoided. This project will explore the conjugation of a range of EGFR-targeting peptides (synthesised using automated solid-phase peptide synthesis) to therapeutically relevant oligonucleotide sequences. Proof-of-concept in vitro assays will enable us to select the peptide that results in enhanced and targeted cell uptake. Conjugation of the selected peptide to oligonucleotides complementary to the 5' untranslated region of EGFR mRNA will enable us to evaluate the therapeutic efficacy of this approach using hypoxic cancer cells. Due to the instability of nucleic acids in plasma, the investigation of strategies to stabilise the peptide-oligonucleotide conjugate will also be critical. Potential avenues of exploration include the use of proteolytically-stable peptide-nucleic acids (PNAs) and formulation of the conjugate into liposomes (lipid nanoparticles) decorated with EGFR-targeting peptides. Any treatment strategy that could allow normal EGFR expression in non-cancerous cells yet produce therapeutic inhibition in diseased cells will reduce side effects and increase maximum tolerated therapeutic doses.