Toni-Christopher Denner

In a bioactivity-guided approach, phytotoxic compounds were isolated from culture extracts of the phytopathogenic fungus Colletotrichum graminicola, leading to the identification of N-(4-hydroxybutyl)-benzenesulfonamide (1). Initial derivatization of 1 resulted in enhanced activity, motivating a structure–activity relationship (SAR) study based on a library of 116 synthetic analogues and the parent compound 1. Phytotoxic effects were evaluated phenotypically using a leaf-spot assay on Arabidopsis thaliana Col-0 and a non-destructive leaf-disk assay based on delayed fluorescence. Exploratory SAR analysis indicated that physicochemical parameters such as molar mass (M), partition coefficient (log P), and the steric bulk of the substituents may contribute to the observed phytotoxic activity. Significant enhancement of phytotoxic activity was achieved through structural optimization. Several derivatives remained active at 0.4 mM, whereas the parent compound was inactive at 15 mM under the same assay conditions.

A small library of 79 substituted phenylsulfonamidoalkyl sulfamates, 1b–79b, was synthesized starting from arylsulfonyl chlorides and amino alcohols with different numbers of methylene groups between the hydroxyl and amino moieties yielding intermediates 1a–79a, followed by the reaction of the latter with sulfamoyl chloride. All compounds were screened for their inhibitory activity on bovine carbonic anhydrase II. Compounds 1a–79a showed no inhibition of the enzyme, in contrast to sulfamates 1b–79b. Thus, the inhibitory potential of compounds 1b–79b towards this enzyme depends on the substituent and the substitution pattern of the phenyl group as well as the length of the spacer. Bulkier substituents in the para position proved to be better for inhibiting CAII than compounds with the same substituent in the meta or ortho position. For many substitution patterns, compounds with shorter spacer lengths were superior to those with long chain spacers. Compounds with shorter spacer lengths performed better than those with longer chain spacers for a variety of substitution patterns. The most active compound held inhibition constant as low as K_i = 0.67 μM (for 49b) and a tert-butyl substituent in para position and acted as a competitive inhibitor of the enzyme.

Various conjugates with rhodamines were prepared by starting with betulinic acid (BA) and platanic acid (PA). The molecules homopiperazine and piperazine, which were identified in earlier research, served as linkers between the rhodamine and the triterpene. The pentacyclic triterpene’s ring A was modified with two acetyloxy groups in order to possibly boost its cytotoxic activity. The SRB assays’ cytotoxicity data showed that conjugates 13–22, derived from betulinic acid, had a significantly higher cytotoxicity. Of these hybrids, derivatives 19 (containing rhodamine B) and 22 (containing rhodamine 101) showed the best values with EC₅₀ = 0.016 and 0.019 μM for A2780 ovarian carcinoma cells. Additionally, based on the ratio of EC₅₀ values, these two compounds demonstrated the strongest selectivity between malignant A2780 cells and non-malignant NIH 3T3 fibroblasts. A375 melanoma cells were used in cell cycle investigations, which showed that the cells were halted in the G1/G0 phase. Annexin V/FITC/PI staining demonstrated that the tumor cells were affected by both necrosis and apoptosis.

Sulfonamides remain an important class of drugs, especially because of their inhibitory effects on carbonic anhydrases. Herein, we have synthesized several sulfonamides and tested them for their inhibitory activity against carbonic anhydrases hCA I, hCA II, hCA IX, and hCA XII, respectively. Thereby, biphenyl- and benzylphenyl-substituted sulfonamides showed high selectivity against hCA IX and hCA XII; these enzymes are common targets in the treatment of hypoxic cancers, and noteworthy inhibitory activity was observed for several compounds toward hCA I that might be of interest for future applications to treat cerebral edema. Compound 3 (4-[3-(2-benzylphenyl)ureido]benzenesulfonamide) held an exceptionally low K_i value of 1.0 nM for hCA XII.

Asiatic acid, a pentacyclic triterpene, was converted into a series of piperazinyl, homopiperazinyl, and 1,5-diazocinyl spacered rhodamine conjugates, differing in the type of spacer and the substitution pattern on the rhodamine moiety of the hybrids. The compounds were tested for cytotoxic activity in SRB assays and compound 12, holding an EC₅₀ of 0.8 nM, was the most cytotoxic compound of this series, but compound 18 (containing a ring expanded 1,5-diazocinyl moiety and n-propyl substituents on the rhodamine) was the most selective compound exhibiting a selectivity factor of almost 190 while retaining high cytotoxicity (EC₅₀ = 1.9 nM, for A2780 ovarian carcinoma).

Acetylated triterpenoids betulin, oleanolic acid, ursolic acid, and glycyrrhetinic acid were converted into their succinyl-spacered acetazolamide conjugates. These conjugates were screened for their inhibitory activity onto carbonic anhydrase II and their cytotoxicity employing several human tumor cell lines and non-malignant fibroblasts. As a result, the best inhibitors were derived from betulin and glycyrrhetinic acid while those derived from ursolic or oleanolic acid were significantly weaker inhibitors but also of diminished cytotoxicity. A betulin-derived conjugate held a K_i = 0.129 μM and an EC₅₀ = 8.5 μM for human A375 melanoma cells.