Investigating Perampanel Antiepileptic Drug by DFT Calculations and SERS with Custom Spinning Cell

Abstract

SERS, a clinical practice where medical doctors can monitor the drug concentration in biological fluids, has been proposed as a viable approach to therapeutic drug monitoring (TDM) of the antiepileptic drug Perampanel. The adoption of an acidic environment during the SERS experiments was found to be effective in enhancing the spectroscopic signal. In this work, we combine SERS experiments, conducted with a custom spinning cell in controlled acidic conditions, with DFT calculations aimed at investigating the possible protonated forms of Perampanel. The DFT-simulated Raman spectra of protonated Perampanel accounts for most of the observed SERS signals, thus explaining the effective role of protonation of the analyte. Our results suggest protonation as a viable approach to fostering SERS of alkaline drugs.

Keywords: noise reduction; quantitative SERS; spinning cell; therapeutic drug monitoring.

Figure 1

Three-dimensional representation of the lowest energy conformation of the Perampanel molecule (see text for details). The coloring scheme is CPK (C: gray, N: blue; O: red; H: white). The possible proton acceptors are the O, N1, N2, and N3 atoms.

Figure 2

SERS spectra of acidic water solution of PER (5 × 10⁻⁵ M) at different pH (see text for details). To help the reader appreciate the effect of PER protonation, we also report the UV-Vis spectra of the same PER solutions taken from previous work [13].

Figure 3

The equilibrium structures of the four protonated forms of PER as determined by DFT (see text for details).

Figure 4

SERS and Raman spectra of Perampanel. From top to bottom: comparison between the results of the experimental dynamic SERS measurements (C1 configuration, see Section 2.2) and the simulated Raman spectra obtained from the sum of the protonated forms of Perampanel (H⁺PER), also including the neutral form (H⁺PER and PER); the spectra of the four protonated forms of Perampanel are also individually reported (H⁺PER_N1, H⁺PER_N2, H⁺PER_N3, and H⁺PER_O). The wavenumber axis of the simulated spectra has been uniformly scaled by the factor of 0.98 to facilitate the comparison with the experimental SERS spectrum. The peak labels in the figure correspond to the respective assignations in Table 2.

Figure 5

SERS spectra of PER (10⁻⁴ M, pH2) measured at a fixed position on a Au SERS pad at increasing laser power (785 nm excitation, collected with 0.1, 1, 10, 100 mW laser power, 50x objective). Signatures of damage are evident at a power of 10 mW. The G and D bands arising due to photodamaging of carbonaceous species are highlighted.

Figure 6

SERS spectra of PER in static mode (10⁻⁴ M, pH 2). Each measurement is taken at a point spaced 5 µm along a line, with 785 nm excitation, laser power 1 mW, 10 s collection time, 2 averages, and 50x objective. The reported spectra are normalized to the intensity of the AuCl peak at 257 cm⁻¹.

Figure 7

SERS spectra of PER (10⁻⁴ M, pH 2) at increasing power, with 785 nm excitation. Spectra collected at 1, 10, 100 mW power with C1 configuration. Up to 100 mW of incident laser power, there is no visible sign of photodamaging.

Figure 8

(a) SERS spectra of PER in C1 mode (10⁻⁴ M, pH 2). Each measurement is taken at a point spaced 5 μm axially, with 785 nm excitation, laser power 25 mW, 10 s collection time, and 2 averages. (b) SERS spectra of PER in C2 mode. Each measurement is taken at a point spaced 5 µm axially, with 785 nm excitation, laser power 1 mW, 10 s collection time, 2 averages, and 50x objective. The reported spectra are normalized to the intensity of the AuCl peak at 257 cm⁻¹.

Figure 9

SEM image of the Au nanoparticles deposited on a Si wafer.

Figure 10

(a) Picture of the device in C1 configuration. (b) Picture of the device in C2 configuration.