Time Analysis of Moisture-Induced Pharmaceutical Transformations Using Raman & DVS

In pharmaceutical drug development, formulations must provide several assurances, including stability, efficacy, and performance. Especially when sensitive to varying environmental factors, active pharmaceutical ingredients (APIs) can be impacted by chemical degradation, physical changes, or diminished therapeutic effect due to moisture and temperature. For example, degradation pathways like hydrolysis can happen faster when there is moisture present, because it acts as a reaction medium that can enhance the breakdown. Also, moisture can act as a plasticizer, which can change the properties of solid formulations and can also impact stability and performance.

Unlocking Stability: The Role of Moisture in Pharmaceutical Performance

The improved analytical method that combines Dynamic Vapour Sorption (DVS) and Raman spectroscopy provides an opportunity to study the effects of moisture on pharmaceutical compounds to trace the changes it induces and adjust the pharmaceutical formulation accordingly.

Compared to conventional methods such as Differential Scanning Calorimetry (DSC) and X-ray Powder Diffraction (XRPD),  which provide information on thermal transitions and crystalline structure, primarily under controlled temperature conditions, this combined method can offer real-time physical and chemical characterization of the changes during moisture sorption. DVS can provide real-time measurements of moisture uptake, while the coupled spectroscopic method provides molecular and structural information associated with moisture absorption. The combined method can provide greater insight into the complexities of moisture-induced chemical and physical changes to pharmaceuticals. Formulation improvements can result when the complex moisture-induced degradation of the formulations is understood.

Moisture interactions cause multifaceted changes to drugs that may be of a physical, chemical, or biological nature. During the first few stages of moisture sorption, there is a process of surface adsorption and physisorption. As the process of moisture sorption continues, there is bulk moisture absorption, which may result in a solid matrix swollen with moisture that undergoes phase transitions or changes in crystallinity. Once moisture is absorbed, the glass transition temperature (Tg)  may decrease in amorphous materials, causing the matrix to gain molecular mobility. During this process, solid-state transformations occur along with the accelerated chemical decomposition of the material. Ultimately, the physical and chemical changes may diminish the potency and stability of the solid-state active pharmaceutical ingredient (API), compromising stability and the intended shelf life of the product.

There are several reasons for understanding moisture interactions:

1. Improved understanding of moisture-induced degradation can support strategies to extend product shelf life and improve stability.
2. Moisture-resistant formulations can be developed through careful selection of excipients and control of solid-state properties.
3. Retention of the physical form and chemical integrity of the API helps ensure the intended therapeutic performance and bioavailability of the drug product.
4. Appropriate environmental controls and moisture-resistant packaging help maintain product stability during storage and handling.

Examining the ways in which moisture is absorbed leads to the discovery of key areas that set the boundary for design controlling conditions to protect the product and predict its shelf life.

Case Studies in the Context of ASA and Citric Acid Monohydrate

This research was conducted on two representative pharmaceutical materials:

1. Acetylsalicylic Acid (ASA), which is moisture-sensitive. It undergoes hydrolytic degradation in the presence of moisture into salicylic acid and acetic acid.
2. Citric Acid Monohydrate is frequently used in pharmaceutical formulations and is also moisture sensitive. It undergoes dehydration and reversible hydrate–anhydrate phase transitions that may alter its physical properties.

The comparison of ASA and citric acid monohydrate is valuable because they represent two distinct moisture-driven transformation pathways: ASA undergoes chemical degradation via hydrolysis, while citric acid monohydrate exhibits reversible solid-state phase transitions (hydrate–anhydrate).

Studying both systems demonstrates that the combined DVS–Raman approach can effectively capture both kinetic (reaction-driven) and thermodynamic (equilibrium-driven) processes, broadening its applicability across pharmaceutical stability challenges.

Key Objective

The primary objective is to understand the changes that take place over a specified time frame under varying temperature and humidity conditions. This study uses DVS and Raman spectroscopy to directly compare moisture-driven transformations in ASA and citric acid monohydrate.

By integrating gravimetric sorption data with real-time molecular spectroscopy, the study aims to distinguish kinetic hydrolytic degradation in ASA from thermodynamically driven hydrate–anhydrate transitions in citric acid monohydrate.

Analytical Techniques

The combination of advanced analytical techniques enables a comprehensive investigation of moisture-driven behavior using Dynamic Vapour Sorption (DVS) in conjunction with Raman spectroscopy:

Dynamic Vapour Sorption (DVS)

A technique that can:

1. Measure the absorption and release of water with high sensitivity.
2. Simulate controlled environmental conditions by controlling the relative humidity (RH) and temperature.
3. Facilitate kinetic measurements of moisture sorption processes.

DVS data can create sorption isotherms and describe the relationship between moisture uptake and relative humidity, and provide profiles of absorption–desorption hysteresis that reveal the moisture sorption behavior of materials.

The kinetic profiles can determine whether crystallization, amorphization, or phase transitions occur, and can also determine if the change is reversible or irreversible. These profiles can provide insight into the stability of a material and how the material may perform under different environmental conditions.

Raman Spectroscopy

1.  Detects molecular and structural transformations by providing molecular vibrational information.
2. Enables identification of chemical transformations such as hydrolysis and solid-state transitions.
3. Correlates molecular spectral changes with gravimetric moisture uptake measured by DVS.

Raman spectroscopy can provide insight into thermally induced molecular and structural changes, as well as phase transitions observed spectroscopically. These phase changes can provide insight into whether transformations arise from changes in crystallinity, chemical composition, or molecular structure.

Many of these transitions, when monitored simultaneously with DVS, can be directly linked to the relative humidity conditions under which they occur.

Instrument Features

1. B&W Tek i-Raman Plus
2. 784.96 nm laser excitation
3. 65–3350 cm⁻¹ spectral range
4. Improved sensitivity and stability from TE-cooled CCD detection

Real-time monitoring of gravimetric mass change alongside the study of the morphology of the sample through moisture absorption

Design of Experiments

ASA Study

1. Climatic chamber set to 90% RH at 25 °C
2. Drying phase temperature increased to 50 °C
3. 60-hour monitoring phase followed by a 24-hour drying phase
4. To observe thermal and chemical structural changes, spectra are taken at a set integration time

This experimental setup is based on accelerated stability testing, whereby elevated temperature and humidity conditions are employed to accelerate degradation processes and evaluate product stability. Moisture-induced hydrolysis, along with structural changes, poses a significant risk to pharmaceutical stability, and studies of this type help define appropriate storage and packaging conditions.

Citric Acid Monohydrate

The stepwise temperature increase study, ranging from 10 °C to 50 °C in 5 °C intervals,  was designed to investigate moisture-induced changes in the sample and identify the relevant thermal range for phase transitions. These studies enable a deeper understanding of dehydration and rehydration behavior.

1. Equilibration and kinetic behavior were monitored for 720 minutes at each temperature step.
2. 0%, 20%, 40%, 60%, and 80% RH were used to investigate moisture-dependent behavior.
3. Raman spectra were collected over 360 minutes to monitor molecular-level structural changes during hydration and dehydration.

DVS data provided detailed sorption isotherms and mass change profiles across the humidity range, highlighting moisture uptake and release behavior at each temperature step. Raman spectra revealed structural changes associated with hydrated and dehydrated forms. This enabled direct comparison between mass changes and molecular-level transformations.

The data were used to determine, with a high level of precision, phase-transition conditions and the reversibility of the transformations under different environmental conditions.

Moisture-Driven Degradation of ASA

1. Hydrolytic degradation pathways can be confirmed through the identification of chemical transformations by Raman spectroscopy.
2. The effects of volatile degradation products were evident, and the apparent rate of volatile product formation increased substantially at elevated temperature and humidity.

These observations confirm that moisture is a major driver of hydrolytic degradation in ASA, particularly under elevated temperature and high relative humidity conditions such as 90% RH at 25 °C.  Additional thermal stress at 50 °C further accelerates degradation.

DVS data suggest that increased water uptake correlates with increased hydrolysis rates. Deviations in sorption behavior and the presence of hysteresis may indicate the onset of phase transitions or structural alterations in the material.

By coupling DVS and Raman spectroscopy data, it is possible to correlate chemical degradation with physical transformations under controlled environmental conditions relevant to pharmaceutical storage and handling.

Monohydrate Dehydration Behavior

1. Temperature and relative humidity determine the transition of monohydrates to anhydrous forms.
2. High humidity requires higher temperatures to dehydrate monohydrates, demonstrating that moisture stabilizes the hydrated form.
3. Solid-state changes can influence dissolution behavior and drug performance.

This illustrates that dehydration depends on both temperature and humidity. DVS showed distinct mass-loss profiles corresponding to different stages of dehydration, while Raman spectroscopy confirmed structural changes between hydrated and anhydrous forms. Controlling environmental conditions is therefore critical for defining formulation stability and predicting product performance under real-world storage conditions.

Improved Drug Stability Predictions through Kinetic Analyses of Transformations

1. Identification of degradation pathways can improve understanding of transformation mechanisms.
2. Characterization of structural and kinetic transformations improves prediction of material stability.
3. Identification of moisture-induced transformations at early stages allows timely intervention.
4. Regulatory compliance is supported by comprehensive evidence of product quality and stability.

 The integrated analytical strategy captures gravimetric sorption and detailed structural changes on a molecular level. This will enhance the ability to guide excipient compatibility studies directly.

It captures moisture-sensitive interactions between APIs and formulation components. It will enhance the ability to select moisture-resistant formulations and offer insight on hygroscopicity and phase behavior to optimize packaging systems. By identifying critical moisture uptake thresholds and environmental sensitivities, it will assist informed decisions on formulation design, choice of materials, and storage strategies and moisture-control conditions.

The integration of DVS and Raman spectroscopy for simultaneous physical and chemical characterization of pharmaceutical solid-state materials can yield novel mechanistic insights into moisture-induced changes that are beyond those provided by traditional analytical methods.

Traditional approaches often yield isolated thermal or structural aspects, whereas this provides a framework to relate sorption behavior to molecular changes in real time. This would enable researchers to relate humidity-induced mass changes to chemical or phase-change transformations in a more holistic manner, improving the overall outcomes of formulations, stability of the final products, and the management of the entire lifecycle of the product.

References:

1. S. Airaksinen et al. "Role of water in the physical stability of solid dosage formulations.", Journal of Pharmaceutical Sciences, 94 10 (2005): 2147-65. https://doi.org/10.1002/JPS.20411.
2. Madhukiran R Dhondale et al. "Co-Crystallization Approach to Enhance the Stability of Moisture-Sensitive Drugs." Pharmaceutics,  15 (2023). https://doi.org/10.3390/pharmaceutics15010189.
3. Veronica, N., Heng, P. W. S., & Liew, C. V. (2022). Ensuring Product Stability–Choosing the Right Excipients. Journal of Pharmaceutical Sciences, 111(8),2158-2171. https://doi.org/10.1016/j.xphs.2022.05.001
4. Ciriminna R., Meneguzzo F., Delisi R., Pagliaro M. Citric acid: Emerging applications of key biotechnology industrial product. Chem. Cent. J. 2017; 11:22. doi: 10.1186/s13065-017-0251-y
5. International Council for Harmonisation – Q1A(R2): Stability Testing of New Drug Substances and Products
6. United States Pharmacopeia – <671> Containers—Performance Testing; Moisture Vapor Transmission
7. European Medicines Agency – Guidelines on Stability Testing of Existing Active Substances and Related Finished Products
8. Stephen R. Byrn et al. – Solid-State Chemistry of Drugs, SSCI Inc.
9. Gordon L. Amidon – Research on moisture effects in solid dosage forms
10. Surface Measurement Systems Ltd – Dynamic Vapour Sorption (DVS) Application Notes
11. Royal Society of Chemistry – Publications on Raman spectroscopy in pharmaceutical analysis
12. FDA – Guidance for Industry: Stability Testing of Drug Substances and Drug Products