Ultrasonic Nano-Crystallization of Salsalate for Improved Bioavailability

Salsalate is part of a class of naturally occurring chemicals known as salicylates, which are among the oldest prescribed medicinal anti-inflammatories [1]. The most common salicylate is acetylsalicylic acid (ASA) (coined Aspirin by Bayer in 1899) and its main biochemical function is the reduction of inflammation and fever via the inhibition of cyclooxygenase enzymes (COX) [2]. Over the last century, it has been suggested that salicylates could have other medicinal benefits, specifically in the treatment of mild cases of diabetes [3, 4, 7]. Aspirin has been administered to patients with pre-diabetes/obesity in order to hinder the evolution of the disease. Unfortunately, inhibition of COX by Aspirin results in abnormalities in bleeding, platelet aggregation and gastric regulation [8, 9]. In retribution, medical practitioners have looked to salsalate [14] to provide treatment to diabetes patients as it has similar anti-inflammatory properties to Aspirin yet has been shown to be significantly less gut damaging [10]. To maximize the bioavailability of the drug and promote effective therapy, nano-crystallization of salsalate clusters must be performed to reduce their median particle size below one micron [12].

High-intensity ultrasonic liquid processing provides a simple means for such particle comminution. In this post we demonstrate how salsalate crystals can be brought to sub-micron median particle sizes by ultrasonic milling with Industrial Sonomechanics bench-scale processor, BSP-1200, configured in the flow-through mode.

Materials and Methods

The ultrasonic nano-crystallization process was conducted using:

Ultrasonic processor: BSP-1200 configured in the flow-through mode
Incorporated horn: Half-wave Barbell horn, amplitude = 80 µm, frequency = 20 kHz
Active ingredient: salsalate, initial median particle size, d₅₀≈ 72 µm
Medium: distilled water
Surfactant: Tween 80
Co-surfactant: isopropyl alcohol
Peripherals: magnetic stir-plate, pump, storage tank, microscope, particle size analyzer

Two samples were prepared as shown below:

	Sample 1		Sample 2
	Mass [g]	Mass Percentage [%]	Mass [g]	Mass Percentage [%]
Water	965	96.5	985	98.5
Tween 80	5	0.5	5	0.5
Isopropyl Alcohol	20	2.0	0	0
Salsalate	10	1.0	10	1.0

Table 1. Salsalate sample components.

Small-chain alcohols (e.g., isopropyl alcohol) are commonly used in pharmaceutical nano-crystallization processes to increase the compatibility of water and the hydrophobic active to be nano-crystalized [5, 6]. To test the preceding claim and determine how the presence of a small-chain alcohol affects the final salsalate particle size, Sample 1 contained 2 % of isopropyl alcohol while Sample 2 was prepared without it.

The samples were premixed on a magnetic stir plate and recirculated through the reactor chamber without sonication for 10 minutes. Ultrasonic output was then initiated, and each sample was processed continuously for 4.5 hours. Both the transducer and reactor chamber were cooled with chilled water (~10 °C), maintaining the processed liquid temperature at 25 °C.

Figure 1. Schematic of BSP-1200 in the flow-through mode.

Results and Discussion

Particle size analyses were performed on the unprocessed sample (see below), Sample 1 (see below) and Sample 2 (data not shown) by Laser Diffraction (Nanotrac, Microtrac Inc., Montgomeryville, PA).

PSD - salsalate.jpg Figure 2. Particle size distribution before (blue) and after (red) sonication of Sample 1.

The blue line (A) shows the particle size distribution for the unprocessed sample, which had the median particle diameter d₅₀ = 72.02 µm. As pictured in the superimposed microscopic image, the unprocessed sample showed large particles of varying sizes. The red line (B) corresponds to the particle size distribution of Sample 1, for which the median particle diameter was ultrasonically reduced to d₅₀ = 0.499 µm. For comparison, the median particle diameter of the processed Sample 2 was higher: d₅₀ = 0.615 µm, which can be attributed to the absence of isopropyl alcohol in this formulation.

Aside from the overall reduction in d₅₀, it is evident that ultrasonic exposure promotes a near uniform distribution of particle sizes. This observation is qualified by the steepness of line B and that nearly all processed salsalate particles achieved a particle diameter in the sub-micron regime (d₈₄= 1 µm).

References

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Reid, James, et al. “Aspirin and Diabetes Mellitus.” British Medical Journal, 9 Nov. 1957, pp. 1071–1074.
Williamson, R. T. “On the Treatment of Glycosuria and Diabetes Mellitus with Sodium Salicylate.” British Medical Journal, vol. 1, no. 2100, 30 Mar. 1901, pp. 760–762.
Burlage, Henry M., and Doris Bullard Hawkins. “Pharmaceutical Applications of Isopropyl Alcohol. I. As a Solvent in Pharmaceutical Manufacturing .” Journal of the American Pharmaceutical Association (Scientific Ed.), vol. 35, no. 12, 1946, pp. 379–384.
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Vaughn, Charles. Use of Salsalate with or without Caffeine and with or without Omega 3, and Other Pharmaceutical Compounds in a Distinctively Unique Nano-particulate Capsule and Tablet. Charles Vaughn, assignee. Patent US 20110064813 A1. 17 Mar. 2011
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Goldfine, Allison B., et al. “Use of Salsalate to Target Inflammation in the Treatment of Insulin Resistance and Type 2 Diabetes.” Clinical and Translational Science, vol. 1, no. 1, May 2008, pp. 36–43.

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