## 1 Introduction  
Pulmonary drug delivery has emerged as a vital approach for treating respiratory and systemic diseases due to the lung's unique physiological features. Direct drug administration to the lungs offers significant advantages, including avoidance of first-pass metabolism, rapid therapeutic onset, and localized drug action, which minimizes systemic side effects [1]. Furthermore, non-invasive administration enhances patient compliance, making this route particularly appealing for chronic conditions such as asthma, chronic obstructive pulmonary disease (COPD), tuberculosis, and cystic fibrosis (CF) [2, 3, 4, 5, 6]. Among the various inhalation devices, dry powder inhalers (DPIs) are preferred due to their portability, ease of use, and the solid-state stability of the formulations [7]. Traditionally, carrier-based DPIs, which rely on excipients like lactose for drug dispersion, have faced limitations in achieving efficient drug deposition in the respiratory tract [8]. To address these challenges, carrier-free DPI systems have been developed, incorporating active pharmaceutical ingredients (APIs) and excipients into a single, cohesive formulation [9]. Particle size and aerodynamic properties play critical roles in the success of pulmonary formulations, with particles in the range of 1-5 \(\upmu\)m being essential for deep lung deposition. Particle engineering plays a pivotal role in DPI development by tailoring the surface properties of drugs, carriers, or excipients. This approach balances interparticle forces, enhances stability during processing and storage, and improves aerodynamic performance [10]. Wet media milling is a well-established pharmaceutical technique for producing stable nanosuspensions of poorly water-soluble APIs. The scalability of wet milling can be enhanced using modeling techniques to optimize industrial applications, reducing costs and improving efficiency [11]. Spray drying, another scalable and efficient particle engineering technique, has been employed to produce inhalable powders with favorable properties, including ease of use and extended shelf-life stability [12]. Recently, combining these two approaches has garnered attention for developing DPI formulations. Stability is a fundamental consideration in pharmaceutical product development, especially for inhalable formulations like DPIs, where storage conditions can significantly influence particle morphology and aerodynamic performance [13]. While solid dosage forms generally exhibit better stability than liquid formulations, nanosized APIs in solid forms are susceptible to aggregation, which can affect their functionality. Crystalline APIs are often preferred for their superior physical stability compared to amorphous forms [14], though amorphous APIs may provide enhanced dissolution and bioavailability. Striking a balance between these properties is essential to address stability challenges such as crystal growth, sedimentation, and agglomeration, ensuring both the safety and efficacy of the final product over its shelf life [15]. Our research group previously developed a carrier-free DPI formulation combining nanosized ketoprofen (KTP) with mannitol (MNT), targeting pulmonary inflammation and improving mucus clearance [16]. By employing wet media milling and spray drying, we successfully produced an optimized formulation with favorable particle size, morphology, and aerodynamic performance. The dual functionality of this formulation allows KTP nanoparticles to address inflammation while MNT enhances mucus clearance. The promising results of this approach warrant further investigations into its pulmonary applicability, particularly its permeability through lung cell lines and long-term stability. Building upon this foundation, the current study aims to extend the investigation of this optimized formulation. Specifically, we sought to evaluate its cytotoxicity, permeability, and stability under stress conditions (40 \({}^{\circ}\)C and 75% RH) as per ICH guidelines.