Ka̅rLis Be̅rZiņš

Poly(N-vinylcaprolactam) (PNVCL) is a thermoresponsive polymer of particular interest for stimuli-responsive drug delivery due to its physiologically relevant lower critical solution temperature (LCST). However, temperature-induced conformational changes vary significantly with polymer concentration, making the polymer’s hydration state a crucial design parameter. In this study, we investigated the thermoresponsive properties of PNVCL at hydration states representative of various drug delivery applications, extending current knowledge on dissolved PNVCL to hydration states also relevant for cutaneous application. PNVCL with low molecular weight (<5000 g/mol) and low dispersity (1.2–1.8) was synthesized via free radical polymerization, and the molecular weight was characterized using a multimethod approach. UV–vis spectrophotometry, Raman spectroscopy, and small- and wide-angle X-ray scattering (SAXS/WAXS) were employed to probe conformational changes at multiple length scales. We found distinct structural transitions around the LCST for all hydration states, consistent with a coil-to-globule transition. The LCST exhibited a significant dependence on hydration state, with transitions occurring at 27 °C for powder PNVCL and at 48 °C for highly diluted PNVCL (10 mg/mL in water). These findings highlight the importance of accounting for concentration-dependent shifts in LCST when designing PNVCL-based drug delivery systems, particularly for applications involving changes in local polymer concentration upon administration.

Dynamic changes in the structure of lipid lyotropic liquid crystals upon mixing of different lipid systems were investigated using simultaneous low-frequency Raman (LFR) and small-angle X-ray scattering (SAXS) at a synchrotron facility. Monoolein and phytantriol were investigated as lipids with broadly highlighted potential for drug delivery. Their dispersed cubic phase structures were perturbed by mixing with a vitamin E acetate emulsion, known to modify the mesophase structures formed by monoolein and phytantriol, in the presence or absence of a lipophilic drug (cinnarizine). This mixing strategy enabled the exploration of structurally distinct and compositionally complex scenarios to establish direct correlations between LFR spectral features and mesophase structural parameters determined by SAXS, thereby assessing how LFR can be used to sensitively track composition-driven mesophase transformations. Crucially, the concurrent LFR-SAXS approach also allowed the evaluation of the affinity for cinnarizine to favorably interact with these lipids and form specific nanostructures leading to increased drug solubilization. These results illustrate how combined LFR-SAXS measurements can inform and expand the future use of LFR as a partially independent, structure-sensitive probe of lipid mesophase dynamics.

Surface-enhanced, low-frequency Raman spectroscopy (SELFRS) was explored for its potential as a structural screening tool within pharmaceutical applications, including facile small-scale multicomponent analysis. Paracetamol was used as the model drug, and its crystallization behavior with or without the presence of a templating agent (benzoic acid) was investigated using commercial silver-based SERS substrates. The Raman imaging was carried out using two different LFR-enabled instruments employing 532 and 785 nm incident lasers, where each of the setups showed certain affinity for differentiating lattice vibrations of the polymorphic forms of interest: form I and form II. A comparison of SELFRS, SERS, and their combination using chemometrics showed the potential for the LFR spectral range to improve surface-enhanced measurements either individually or in combination with the typically-used fingerprint region without the need to alter the experimental configuration. Additionally, the use of crystallization additives that helped to drive the formation of metastable form II was shown using SELFRS to provide additional mechanistic understanding of the template-assisted crystallization processes.

The noninvasive in situ monitoring of the status of drug retention and implant integrity of subcutaneous implants would allow optimization of therapy and avoid periods of subtherapeutic delivery kinetics. A proof-of principle study was conducted to determine the use of microspatially offset low-frequency Raman spectroscopy (micro-SOLFRS) for nonintrusive in situ analysis of subcutaneous drug delivery systems. Caffeine was used as the model drug, and it was embedded in a circular-shape Soluplus matrix via vacuum compression molding. For the exploratory analysis, prototype implants were positioned underneath skin tissue samples, and various caffeine concentrations (1-50% w/w) and micro-SOLFRS displacement settings (Δz = 0-8 mm) were tested from the pseudo three-dimensional (3D)-imaging perspective. This format allowed the optimization of real-time micro-SOLFRS analysis of implants through skin tissue that was embedded in an agarose hydrogel. Notably, this analytical approach allowed the temporal and spatial erosion of the implant and solid-state transformations of caffeine to be distinguished. The spectrometric results correlated with complementary high-performance liquid chromatography (HPLC) determination of changes in drug concentration, illustrating drug dissipation/diffusion characteristics. The discovered capability of micro-SOLFRS for in situ measurements of drugs and implants makes it attractive for biomedical diagnostics that, ultimately, could result in development of a new point-of-care technology.

A low-frequency Raman (LFR) probe was coupled to an in-line small-angle X-ray scattering (SAXS) beamline to test the capabilities of a combinatory approach for the determination of lipid and drug behavior during the enzymatic lipolysis of milk-based oral formulations. Cinnarizine was used as the model drug, and its solubilization dynamics as well as its potential impact on the supramolecular structures formed by the digestion products of bovine milk were evaluated from the perspective of both techniques. The SAXS data were superior in distinguishing various liquid crystalline assemblies formed during the digestion process, with LFR providing complementary information regarding the formation of calcium soaps. On the other hand, studying changes in the LFR domain allowed the differentiation of drug solubilization and precipitation; processes that were less clear from the X-ray scattering data. Given the relative simplicity of the combined experimental setup, these results highlight the advantages that the combination of the two techniques can provide for understanding and developing new lipid-based formulations and will help to translate the results obtained at synchrotron facilities to routine analysis procedures in laboratory/industry-based environments.

Low-frequency Raman spectroscopy (LFRS) is a valuable tool to detect the solid state of amorphous and crystalline drugs in solid dosage forms and the transformation of drugs between different polymorphic forms. It has also been applied to track the solubilisation of solid drugs as suspensions in milk and infant formula during in vitro digestion. This study reports the use of LFRS as an approach to probe drug precipitation from a lipid-based drug delivery system (medium-chain self-nanoemulsifying drug delivery system, MC-SNEDDS) during in vitro digestion. Upon lipolysis of the digestible components in MC-SNEDDS containing fenofibrate as a model drug, sharp phonon peaks appeared at the low-frequency Raman spectral region (−1), indicating the precipitation of fenofibrate in a crystalline form from the formulation. Two multivariate data analysis approaches (principal component analysis and partial least squares discriminant analysis) and one univariate analysis approach (band ratios) were explored to track these spectral changes over time. The low-frequency Raman data produces results in good agreement with in situ small angle X-ray scattering (SAXS) measurements with all data analysis approaches used, whereas the mid-frequency Raman requires the use of PLS-DA to gain similar results. This suggests that LFRS can be used as a complementary, and potentially more accessible, technique to SAXS to determine the kinetics of drug precipitation from lipid-based formulations.

The solid-state landscape of carbamazepine during its dehydration was explored using Raman spectroscopy in the low- (−300 to −15, 15 to 300) and mid- (300 to 1800 cm⁻¹) frequency spectral regions. Carbamazepine dihydrate and forms I, III, and IV were also characterized using density functional theory with periodic boundary conditions and showed good agreement with experimental Raman spectra with mean average deviations less than 10 cm⁻¹. The dehydration of carbamazepine dihydrate was examined under different temperatures (40, 45, 50, 55, and 60 °C). Principal component analysis and multivariate curve resolution were used to explore the transformation pathways of different solid-state forms during the dehydration of carbamazepine dihydrate. The low-frequency Raman domain was able to detect the rapid growth and subsequent decline of carbamazepine form IV, which was not as effectively observed by mid-frequency Raman spectroscopy. These results showcased the potential benefits of low-frequency Raman spectroscopy for pharmaceutical process monitoring and control.