The efficacy of current chemotherapeutic treatments against most solid tumors is limited by their systemic toxicity, which is partly associated with the cytotoxic properties of agents such as docetaxel or doxorubicin. To avoid or minimize adverse effects from chemotherapeutic molecules, a promising targeted approach is through peptide-drug conjugates (PDCs) that link anticancer molecules to peptides designed to interact with receptors highly expressed on cancer cells, and which can mediate the molecules rapid internalization within those cells. One such receptor is sortilin (SORT1), also known as neurotensin receptor3, a membranebound receptor that belongs to the VPS10P family of receptors. TH19P01 peptide was recently designed to target and exploit SORT1s ligand internalization function. Studies have confirmed that both TH1902 (a docetaxel-TH19P01 conjugate) and TH1904 (a doxorubicin-TH19P01 conjugate) require a SORT1-dependent mechanism of action to exert anticancer activities. In recent preclinical studies performed in immunocompromised animal models, which are unable to produce mature T-cells, TH1902 was effective against several human SORT1-positive xenograft models including triple-negative breast cancer (TNBC), ovarian cancer, and endometrial cancer.

The crucial steps during the synthesis of aminooxyacetylated peptides are the incorporation of aminooxyacetic acid (Aoa), including a protecting group, and the final cleavage and the working-up procedure of the Aoa-containing peptide derivatives. In most cases, Boc-protected Aoa is attached to a peptide chain in the last step of solid-phase peptide synthesis. It has been observed that over-acylation (additional Boc-Aoa-OH connected to the Aoa moiety) may occur as the main side reaction. Many approaches have been investigated to overcome this problem, including carbodiimide-mediated one-pot acylation without a base or the application of Boc-Aoa-OSu active ester as an acylating agent, as well as the use of a high excess (8 equiv) of Boc-Aoa-OH and coupling agents for a short acylation time (10 min). Nevertheless, the coupling of the diBoc-protected Aoa derivative has proved to be the best solution. However, the aminooxyacetyl moiety is very sensitive to molecules containing carbonyl groups, with the partial impact of the peptide sequence. Therefore, the free NH2-O-R group reacts often with these compounds during the working-up procedure after the final cleavage of Aoa-modified peptides from the resin. These carbonyl group-containing derivatives might come from the plastic tubes or residues of acetone used in a laboratory. This cannot be prevented even by using diBoc-protected Aoa or working in argon. We found a highly sensitive peptide to this side reaction; the synthesis of a somatostatin analog developed in Schallys laboratory elongated with Aoa (H-Aoa-D-Phe-c[Cys-Tyr-D-Trp-Lys-Val-Cys]-Thr-NH2) was unsuccessful. After several trials to optimize the reaction conditions, we elaborated the following procedure: The semi-protected peptide H-D-Phe-Cys-Tyr-D-Trp-Lys(Dde)-Val-Cys-Thr-NH2 was cleaved from Rink Amide MBHA resin and reacted with Boc-Aoa-OPcp to incorporate Aoa into the N-terminus in a solution. After the efficient coupling reaction, the Dde-protecting group was removed with 2% hydrazine in DMF. Surprisingly, during the cleavage of Dde, a cyclic peptide also formed (Boc-Aoa-D-Phe-c[Cys-Tyr-D-Trp-Lys-Val-Cys]-Thr-NH2). The Boc group was cleaved in 95% TFA solution in the presence of 10 equiv-free Aoa as a carbonyl capture reagent that could prevent the reaction of the peptide with any carbonyl derivative. The crude product was purified by RP-HPLC, and the solvent was evaporated, followed by direct ligation to daunomycin (Dau) in 0.2 M NaOAc solution at pH 5. This procedure proved to be very efficient to prepare oxime-linked Dau-peptide conjugates.