Antibacterial Activity of Some 5-Dialkylaminomethylhydantoins and Related Derivatives

Abstract

In connection with our studies on antibacterial compounds in the class of 5-dialkylaminomethylhydantoins against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) strains, some molecular modifications were attempted. The antibacterial activities of all of the synthesized hydantoin derivatives were evaluated. Among the hydantoin derivatives designed in this study, C₂-symmetrical twin-drug type compound (7) showed the highest level of antibacterial activity against S. aureus strain.

Since bacteria can develop resistance to generally used antibacterial agents, resulting in the generation of various drug-resistant bacteria strains, the development of new antibacterial agents is required. In the early invasion stage of bacteria such as Escherichia (E.) coli, cell surface glycans of bacteria recognize host cell lectin. For such recognition by glycans, the major recognition patterns between the host and guest molecules are thought to be intermolecular hydrogen bonding interactions. This interaction process is a logical path and is thought to control biological response. We have been interested in molecules that interfere with such carbohydrate recognition stages in order to find new antibacterial leads.

In connection with our studies on antibacterial compounds, we have already reported some molecular designs, synthesis and biological evaluations of a few heterocyclic compounds or various types of functionalized symmetrical molecules.^1–5) In our previous articles on chemical modifications of linezolid, we reported synthesis and antibacterial activity of new structural isomers of oxazolidin-2-ones as a general structure (A)³⁾ or a bioisosteric replacement⁶⁾ of the oxazolidinone ring in linezolid⁷⁾ by a hydantoin nucleus (B) (Fig. 1). Among previously synthesized target hydantoin derivatives (B), we found that many 5-dialkylaminomethylhydantoin derivatives showed a wide range of remarkable antibacterial activities (minimum inhibitory concentration (MIC) values in a sub-micro molar range) against both Staphylococcus (S.) aureus and E. coli strains. This finding encouraged us to investigate further molecular modifications of this class of compounds. We therefore carried out additional synthetic investigations of newly designed 5-dialkylaminomethylhydantoin derivatives.⁸⁾

Fig. 1. Structures of A, B and Linezolid

In this article, we present results of biological evaluation for antibacterial activity of these new 5-dialkylaminomethyl-3-substituted hydantoin derivatives with E. coli and S. aureus strains.

Antibacterial Activity and Discussion

Many tested hydantoin derivatives showed significant antibacterial activity against both E. coli (Gram-negative) and S. aureus (Gram-positive) at concentrations of 0.024–0.226 mM.

Among the 3-(2-chloroethyl)-substituted new hydantoin derivatives (3a, b, 4, 5) listed in Table 1, compound (3a) showed the highest antibacterial activities against both of E. coli and S. aureus strains (MIC=0.113, 0.226 mM, respectively), but the levels of its activity were lower than those of 3-p-chlorophenyl-substituted derivatives (3f)⁶⁾ reported previously. 3-Chloroethyl-5-disubstituted hydantoin (4) did not show any remarkable antibacterial activity against either E. coli and S. aureus strains at a concentration of 0.513 mM. The deamination product (5) showed a weaker antibacterial activity (MIC=0.367 mM) only against E. coli than those of 3a and 3b (see Table 1).

Table 1. Antibacterial Activities of Target Hydantoin Derivatives (3–7)

Compound	Antibacterial activity MIC (mM)		Compound	Antibacterial activity MIC (mM)
	E. coli	S. aureus		E. coli	S. aureus
	0.113	0.226		0.216	>0.432
	>0.513	>0.513		0.367	>0.733
	>0.344	>0.344		>0.332	>0.332
	>0.364	0.091		0.048	0.097
	0.044	0.088		0.104	>0.416
	>0.366	>0.366		0.190	0.190
	>0.206	0.051		0.095	0.024

a) Data were taken from ref. 6.

N(1)-Unsubstituted hydantoin derivatives (3c–e) that have a sterically bulky aryl ring such as a biphenyl or naphthyl group at the N(3)-position of the hydantoin ring generally showed lower levels of activity than those of 3-chlorosubstituted phenyl derivatives (3f, g) except for compound 3e. Compound 3e showed notable antibacterial activity against S. aureus (MIC=0.091 mM). Through these structural modifications by the introduction of a sterically bulky aromatic ring, no significant potentiating effect of antibacterial activity was observed.

Among the compounds described in this paper, it is noteworthy that the introduction an N(1)-methyl group on the hydantoin nucleus resulted in decreases in antibacterial against both E. coli and S. aureus. For example, compound 3f showed an MIC value of 0.048 mM against E. coli, but the corresponding N(1)-methylated derivative 3h had decreased activity (MIC=0.104 mM). A similar decrease in activity to S. aureus was also observed. This peculiarity of structure indicates that the N(1)-H moiety in the hydantoin ring in the molecule plays an important role in the action for antibacterial activity. Compared with the structure of linezolid, we have pointed out in our previous paper that the bioisosteric hydantoin nucleus⁶⁾ has a characteristic functional group (N-H) for acting as a hydrogen bonding donor in the 5-membered hydantoin ring system. The results obtained for the new N(1)-methyl hydantoin derivatives (3h–j) by a bioisosteric modification seem to provide useful information regarding the importance of the presence of a functional group acting as a hydrogen bonding donor in the molecule for antibacterial activity.

Symmetrical twin-drug type molecules,^9,10) in general, are expected to produce more potent and/or selective biological activities stemming from the ligand pharmacophore. The two identical ligands could bind to the same receptor or enzyme sites simultaneously. From this well-known conception of identical twin-drugs, further molecular modifications of this class of hydantoins to twin-drug type molecules seemed to be interesting in the search for new leads. We therefore attempted synthesis and antibacterial evaluation of a twin-drug type molecule (7) having a diphenylmethane moiety as a linker (see Table 1). The C₂-symmetrical compound 7¹¹⁾ showed the highest level of antibacterial activity (MIC=0.024 mM) against S. aureus and also a high level antibacterial activity against E. coli (Gram-negative) (MIC=0.095 mM).

The intermediate C₂-symmetrical urea derivative (6) as a precursor to the compound (7) also showed significant antibacterial activity (MIC=0.051 mM) only against S. aureus.

As structural factors for antibacterial activities in this series, the results obtained suggest not only the state of an electric charge of an introduced aryl ring but also the contribution of a rotational isomer⁸⁾ about the C–N pivot bond connecting the two ring systems (see 3e or 3j). The results for 2-chloroethyl derivatives (3a, b) at the N(3) position of a hydantoin ring also seem to indicate the importance of a substituent at the N(3) position in the hydantoin ring together with the nature of substituents such as lipophilic properties.

At present, we have no clear evidence that these types of hydantoin derivatives have the same mode of antibacterial action as that of linezolid. However, to obtain some identical information on the mode of antibacterial activity for the target molecules as well as to confirm the usefulness of this hydantoin scaffold for antibacterial efficacy, further synthetic trials of new hydantoin derivatives including new types of C₂-symmetrical twin-drug type derivatives containing other linkers are being conducted. Regarding sugar recognition properties of highly bioactive twin-drug type compounds such as 7 and 6, we are also planning to carry out calorimetric experiments with sugar derivatives.

Experimental

Synthesis of Target 5-Dialkylaminomethyl-3-substituted Hydantoins and Related Compounds

According to the procedure reported in our previous paper,⁸⁾ we prepared target 5-dialkylaminomethyl-3-aryl-hydantoins (3) by cyclization of urea intermediates (2), which are easily obtained from the reaction of β-aminoalanine ester derivatives (1) with isocyanates. Compound (7) was also prepared by double cyclization of symmetrical intermediate (6) as shown in Chart 1. The structures of these compounds have been shown in detail in our previous paper. Compounds (3a, c, e, 7) were used as hydrochlorides for antibacterial activity assay.

Chart 1. Synthesis of 5-Dialkylaminomethylhydantoins 3 and 7

Assay for Antibacterial Activity

We used S. aureus ATCC6538P and E. coli NBRC14237 (NIHJ) (Gram-positive and Gram-negative bacteria, respectively) as target organisms. Synthesized compounds (3–7) were dissolved in dimethyl sulfoxide (DMSO) to a concentration of 1.280 µg/mL. The minimum inhibitory concentration (MIC) of a standard strain was measured by the authentic microdilution method to monitor the bacterial growth turbidity in Muller–Hinton broth according to the Japanese Society of Chemotherapy.^12,13) The values of MIC for target compounds determined by this authentic MIC method, as well as those of previously reported compounds (3f, g) for comparison, are summarized in Table 1. The values of MIC are expressed as molar concentrations (mM) for the discussion of structure–activity relations.

References

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