Synthesis of multi ring-fused imidazo [1,2- a]isoquinoline-based fluorescent scaffold as anti-Herpetic agent

Abstract

Background: Natural product-inspired synthesis is a key incorporation in modern diversity-oriented synthesis to yield biologically novel scaffold. Inspired by β-carboline fused system, we have designed molecules with multi ring fused scaffold by modifying the tricyclic pyrido[3,4- b]indole ring with imidazo[1,2- a]isoquinoline.

Methods: A highly convergent approach with new C-N and C-C bond formation to synthesize multiring fused complex scaffold imidazo[1,2- a]isoquinolinies as fluorophores. N-nucleophile-induced ring transformation of 2 H-pyran-2-one followed by in situ cis-stilbene-type oxidative photocyclization yielded new C-C bond formation without additional oxidant. The cytotoxicity, effective concentrations, and the mode of action of the synthesized analogs were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT),, plaque reduction, time of addition, and reverse transcriptase Polymerase Chain Reaction (PCR).

Results: Novel imidazo[1,2- a]isoquinoline analogs were prepared, and the results revealed that trans isomer of cyclopropyl analog (EC₅₀ 35 and 37.5 µg/ml) and trans isomer of citric acid salt of phenyl analog (EC₅₀ 38.2 and 39.8 µg/ml) possess significant anti-Herpes Simplex Virus (HSV) activity with selectivity index of >10. The kinetic study demonstrated that both the analogs inhibited HSV-1F and HSV-2G at 2-4 h postinfection. Finally, western blot and reverse transcriptase PCR assays revealed that both the analogs suppressed viral immediate early transcription.

Conclusion: Novel imidazo[1,2- a]isoquinoline analogs were synthesized from pyranone with appropriate amines. Two compounds showed better antiviral profile on HSV-infected Vero cells, compared to the standard drug acyclovir (ACV). Overall, we discovered a promising scaffold to develop a nonnucleoside lead targeting the viral immediate early transcription for the management of HSV infections.

Keywords: 2-]isoquinoline; Imidazo[1; herpes simplex virus; immediate early transcription.

Figure 1.

ORTEP diagram of [3b] (crystallized in ethyl acetate: n-hexane 30:70) showing the X-ray molecular structure at 30% probability level.

Scheme 1.

Synthesis of key intermediates [1a–d]. Reagents and conditions: (i) Toluene, 120°C, 24 h. Synthesis of [3a–h]. Reagents and conditions: (ii) NaH, THF, 24°C rt, 4 h; (iii) hυ, O₂ 48 h.

Figure 2.

(a) Absorbance and emission spectra of [3a] and (b) Fluorescence spectra of [3a], [3c], and [3e] in 1 μM solution at pH 7.

Figure 3.

Time of addition assay of [3g] and [4b] against HSV-1F (a) and HSV-2G (b). Vero cells were infected with HSV-1F (a) and 2G (b), untreated or treated with [3g] or [4b] or ACV at their EC100 concentrations at various time points: preinfection (3 h, 1 h), coinfection (0 h), and postinfection (1–24 h) were subjected to the plaque reduction assay. Each bar represents the mean ± SEM of three independent experiments. (c) Effect of [3a] and [3b] on IE gene expression: HSV-1 infected (5 m.o.i.) Vero cells were treated with [3g] and [4b] for 2–8 h postinfection. Then RNA was isolated and subjected to cDNA synthesis, followed by the quantitative reverse transcriptase PCR of ICP4 and ICP27, using GAPDH as control. GAPDH: Glyceraldehyde 3-phosphate dehydrogenase; HSV: Herpes Simplex Virus; ICP: Infected Cell Polypeptide; PCR: Polymerase Chain Reaction.