Suitability of CysWhite™ (Cysteamine HCL) in Leave-on Melasma Serum

Based on the information about the ingredients, the proposed formulation for a leave-on facial serum is not suitable, primarily due to the recommended usage of CysWhite™ (Cysteamine HCL).

Here's a breakdown:

• CysWhite™ (Cysteamine HCL): This ingredient is highly effective for reducing melasma, but it has specific usage instructions. The description states that it should be washed off the skin after applying for 15 minutes, and should not be left on for more than 30 minutes. This makes it unsuitable for a typical leave-on serum formulation. Additionally, CysWhite™ has low stability when dissolved in water. For a long shelf-life in a water-based formula, it's recommended to include ActiveProtec™ OX, which is not in your proposed formula.
• Ethyl Ascorbic Acid: This is a stable, water-soluble Vitamin C derivative suitable for skin brightening. The proposed concentration (3%) is within the recommended range. For optimal stability and effectiveness in a water-based formula, the pH should be between 3.5-6.0, and it's recommended to include 0.2% Disodium EDTA. Your formula does not include Disodium EDTA, and the final pH would need to be adjusted and checked.
• Odor-Kill™: Including Odor-Kill™ is appropriate for helping to mask the strong odor of CysWhite™, and your proposed concentration (2%) is within the recommended range for odor absorption.

Conclusion:

While the individual concentrations of the ingredients are within their recommended ranges, mixing CysWhite™ into a leave-on serum base is not advisable because CysWhite™ is designed for short-contact therapy (wash-off). Leaving it on the skin for extended periods is not recommended based on the product information and could potentially lead to irritation or other issues.

A formulation using CysWhite™ for melasma should be designed as a wash-off treatment. For a leave-on serum targeting melasma, other ingredients like Ethyl Ascorbic Acid (with appropriate pH adjustment and stabilizers), Tranexamic Acid, or Niacinamide might be more suitable.