Preparation of Thermocleavable Conjugates Based on Ansamitocin and Superparamagnetic Nanostructured Particles by a Chemobiosynthetic Approach

Abstract

A combination of mutasynthesis, precursor-directed biosynthesis and semisynthesis provides access to new ansamitocin derivatives including new nanostructured particle-drug conjugates. These conjugates are based on the toxin ansamitocin and superparamagnetic iron oxide-silica core shell particles. New ansamitocin derivatives that are functionalized either with alkynyl- or azido groups in the ester side chain at C-3 are attached to nanostructured iron oxide core-silica shell particles. Upon exposure to an oscillating electromagnetic field these conjugates heat up and the ansamitocin derivatives are released by a retro-Diels-Alder reaction. For example, one ansamitocin derivative exerts strong antiproliferative activity against various cancer cell lines in the lower nanomolar range while the corresponding nanostructured particle-drug conjugate is not toxic. Therefore, these new conjugates can serve as dormant toxins that can be employed simultaneously in hyperthermia and chemotherapy when external inductive heating is applied.

Synthesis of Magnetic‐Nanoparticle/Ansamitocin Conjugates—Inductive Heating Leads to Decreased Cell Proliferation In Vitro and Attenuation Of Tumour Growth In Vivo

“…Neither ansamitocin derivative 5 d nor other low‐molecular‐weight degradation products could be detected in either media, irrespective of the presence or absence of serum suggesting that the presence of serum in the media does not affect the stability of the conjugate. A first insight into the biological activity of ansamitocin derivative 5 d was gained by determining the antiproliferative activity (IC 50 [nmol L −1 ]) against multiple mammalian cancer cell lines [L‐929=4.9 (0.2 for AP3 1 ), KB‐3‐1=0.41 (0.17), PC‐3=2.2 (0.06), SK‐OV‐3=1.6 (0.05) and A‐431=0.77 (0.08)] as previous reported . In fact, it shows similar activity as ansamitocin P3 ( 1 ), a prerequisite for the drug‐delivery system described here.…”

“…The preparation of the conjugates 8 c and 8 d was recently reported from our laboratories . It relies on precursor directed biosynthesis (PDB) using an AHBA(−) mutant strain of Actinosynnema pretiosum (strain HGF073), the producer of the ansamitocins .…”

“…We therefore had to utilise two different generator/inductor devices in the present study (for more information see Table and the Supporting Information). Both devices are equipped with high‐frequency inductors (650–1000 kHz), since medium‐frequency inductors were inefficient to generate enough heat inside small samples of the MAGSILICA® nanostructure particles 8 d in order to efficiently release the toxin 5 d into the aqueous medium ,…”

A Bio‐Chemosynthetic Approach to Superparamagnetic Iron Oxide–Ansamitocin Conjugates for Use in Magnetic Drug Targeting

“…Structurally, the ansamitocins only differ in the ester side chain at C-3 and this position is very tolerant to structural change. [8,9,10] Their antimitotic mode of action is based on their interaction with β-tubulin, preventing polymerization, and thereby arresting cell cycle progression. Despite phase II clinical studies almost four decades ago, they only recently entered the clinic as "warheads" in tumor-targeted immunoconjugates.…”

“…However, we only continued with ansamitocin derivative 16, because it can be prepared more straightforwardly when large amounts of the corresponding conjugate are required. It was immobilized on maleimide functionalized MAGSILICA ® 18 [10] However, it must be noted that maleimide-functionalized particle 18 tends to bind ansamitocin derivative 16 by adsorption. This results in nanostructured particles with cytotoxic properties.…”