Drug delivery systems in oncology for treating cancer are based on different drug release mechanisms at the tumor site including primarily hydrolytic, reductive, enzymatic and/or acid-sensitive cleavage. Drug carrier conjugates that incorporate an acid-sensitive breaking-point exploit the extra- and intracellular acidic environment of the tumor. Important requirements for acid-sensitive bonds are high stability of the carrier-bound drug in the blood circulation and an effective or sustained release of the active drug in the acidic tumor interstitium and acidic endosomes/lysosomes of tumor cells. In addition, sufficient stability of the acid-sensitive bond is a prerequisite for galenic formulation and reconstitution. As a continuation of our development of aldoxorubicin, an acid-sensitive albumin-binding drug of doxorubicin, which is being clinically assessed in advanced clinical trials (www.cytrx.com), we have set out to design a novel acid-sensitive drug release platform that is applicable to carriers such as serum proteins and antibodies.

The key components of this specific drug delivery system are linkers containing a thiol-binding maleimide group and a substituted aromatic hydrazine to form an acid-cleavable hydrazone bond with suitable carbonyl-containing drugs. For establishing and fine-tuning the pH-dependent release profiles, the aromatic moiety of the linker was substituted with a spectrum of electron-withdrawing groups, and reacted with the anthracyclines doxorubicin and nemorubicin. The resulting hydrazone derivatives were conjugated to the cysteine-34 position of albumin as a model protein. Subsequently, the drug release was determined at physiological and acidic conditions as well as in human plasma. It was discovered that by variation of the electron-withdrawing groups, the pH dependent release of the drug could be substantially varied between 1-50 h in the pH range of 4-5. The technology using these linkers was coined LADR™ (Linker Activated Drug Release) and has the additional advantage that the galenic formulation and reconstitution of LADR™-based drugs is facilitated. We have applied the LADR™ technology to several anticancer drugs such as vinblastine and gemcitabine. In summary, LADR™ is an innovative and versatile linker technology creating a platform for developing acid-sensitive drug carrier conjugates that allow the carrier-bound drug to be released in a controlled manner resulting in sustained exposure to cancer cells.

Citation Format: Khalid Abu Ajaj, Stephan David Koester, Friederike Inga Nollmann, Simon Waltzer, Olga Fuchs, André Warnecke, Felix Kratz. LADR™: A novel linker activated drug release technology for drug delivery. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4858.