The Interplay between Mutasynthesis and Semisynthesis: Generation and Evaluation of an Ansamitocin Library

First Ring-Expanded Maytansin Lactone Accessed by a New Mutasynthetic Variant

A multiblocked mutant strain (ΔAHBA and Δasm12, asm21) of Actinosynnema pretiosum, the producer of the highly toxic maytansinoid ansamitocin, has been used for the mutasynthetic production of new proansamitocin derivatives. The use of mutant strains that are blocked in the biosynthesis of an early building block as well as in the expression of two tailoring enzymes broadens the scope of chemo-biosynthetic access to new maytansinoids. Remarkably, a ring-expanded macrolactone derived from ansamitocin was created for the first time.

New geldanamycin derivatives with anti Hsp properties by mutasynthesis

Mutasynthetic supplementation of the AHBA blocked mutant strain of S. hygroscopicus, the geldanamycin producer, with 21 aromatic and heteroaromatic amino acids provided new nonquinoid geldanamycin derivatives. Large scale (5 L) fermentation provided four new derivatives in sufficient quantity for full structural characterisation. Among these, the first thiophene derivative of reblastatin showed strong antiproliferative activity towards several human cancer cell lines. Additionally, inhibitory effects on human heat shock protein Hsp90α and bacterial heat shock protein from H. pylori HpHtpG were observed, revealing strong displacement properties for labelled ATP and demonstrating that the ATP-binding site of Hsps is the target site for the new geldanamycin derivatives.

Heat Shock Proteins Revisited: Using a Mutasynthetically Generated Reblastatin Library to Compare the Inhibition of Human and Leishmania Hsp90s

Thirteen new reblastatin derivatives, with alkynyl, amino and fluoro substituents on the aromatic ring, were prepared by a chemo-biosynthetic approach using an AHBA(-) mutant strain of Streptomyces hygroscopicus, the geldanamycin producer. The inhibitory potencies of these mutaproducts and of an extended library of natural products and derivatives were probed with purified heat shock proteins (Hsps), obtained from Leishmania braziliensis (LbHsp90) as well as from human sources (HsHsp90). We determined the activities of potential inhibitors by means of a displacement assay in which fluorescence-labelled ATP competes for the ATP binding sites of Hsps in the presence of the inhibitor in question. The results were compared with those of cell-based assays and, in selected cases, of isothermal titration calorimetry (ITC) measurements. In essence, reblastatin derivatives are also able to bind effectively to the ATP-binding site of LbHsp90, and for selected derivatives, moderate differences in binding to LbHsp90 and HsHsp90 were encountered. This work demonstrates that parasitic heat shock proteins can be developed as potential pharmaceutical targets.

Evaluation of the Synthetic Potential of an AHBA Knockout Mutant of the Rifamycin Producer Amycolatopsis mediterranei

Supplementing an AHBA(-) mutant strain of Amycolatopsis mediterranei, the rifamycin producer, with a series of benzoic acid derivatives yielded new tetraketides containing different phenyl groups. These mutasynthetic studies revealed unique reductive properties of A. mediterranei towards nitro- and azidoarenes, leading to the corresponding anilines. In selected cases, the yields of mutaproducts (fermentation products isolated after feeding bacteria with chemically prepared analogs of natural building blocks) obtained are in a range (up to 118 mg L(-1)) that renders them useful as chiral building blocks for further synthetic endeavors. The configuration of the stereogenic centers at C6 and C7 was determined to be 6R,7S for one representative tetraketide. Importantly, processing beyond the tetraketide stage is not always blocked when the formation of the bicyclic naphthalene precursor cannot occur. This was proven by formation of a bromo undecaketide, an observation that has implications regarding the evolutionary development of rifamycin biosynthesis.

New, non-quinone fluorogeldanamycin derivatives strongly inhibit Hsp90

Streptomyces hygroscopicus is a natural producer of geldanamycin. Mutasynthetic supplementation of an AHBA-blocked mutant with all possible monofluoro 3-aminobenzoic acids provided new fluorogeldanamycins. These showed strong antiproliferative activity and inhibitory effects on human heat shock protein Hsp90. Binding to Hsp90 in the low nanomolar range was determined from molecular modelling, AFM analysis and by calorimetric studies.

Preparation of new alkyne-modified ansamitocins by mutasynthesis

The preparation of alkyne-modified ansamitocins by mutasynthetic supplementation of Actinosynnema pretiosum mutants with alkyne-substituted aminobenzoic acids is described. This modification paved the way to introduce a thiol linker by Huisgen-type cycloaddition which can principally be utilized to create tumor targeting conjugates. In bioactivity tests, only those new ansamitocin derivatives showed strong antiproliferative activity that bear an ester side chain at C-3.

Bioreduction of aryl azides during mutasynthesis of new ansamitocins

Supplementing a culture of a mutant strain of Actinosynnema pretiosum that is unable to biosynthesize aminohydroxy benzoic acid (AHBA), with 3-azido-5-hydroxy-benzoic acid and 3-azido-5-amino-benzoic acid, unexpectedly yielded anilino ansamitocins instead of the expected azido derivatives. This is the first example of the bioreduction of organic azides. The unique nature of these results was demonstrated when 3-azido-5-amino-benzoic acid was fed to the corresponding AHBA blocked mutant of Streptomyces hygroscopicus, the geldanamycin producer. This mutasynthetic experiment yielded the fully processed azido derivative of geldanamycin.

Unprecedented deoxygenation at C-7 of the ansamitocin core during mutasynthetic biotransformations

We describe the unprecedented formation of six ansamitocin derivatives that are deoxygenated at C-7 of the ansamitocin core, obtained during fermentation experiments by employing a variety of Actinosynnema pretiosum mutants and mutasynthetic approaches. We suggest that the formation of these derivatives is based on elimination at C-7/C-8 followed by reduction(s) of the intermediate enone. In bioactivity tests, only ansamitocin derivatives bearing an ester side chain at C-3 showed strong antiproliferative activity.