Topical Analgesics: Critical Issues Related to Formulation and Concentration

Topical analgesics are in need to be differentiated from transdermal formulations of analgesics. Topical analgesics are characterized by local analgesic effects in the absence of systemic effects, and do not require a transdermal delivery formulation. There are two key issues in the development of topical analgesics. 1. For optimal clinical effects specific characteristics for the vehicle (a cream base or gel base) are required, depending on the physicochemical characteristics of the pharmaceutical active ingredient in the carrier. 2. One cannot and should not skip well designed phase II dose-finding studies, and this unfortunately happens often, as we will discuss in this paper. In fact, we will demonstrate underdosing is one of the major hurdles to detect meaningful and statistically relevant clinical effects of topical analgesics. In the case of gels or creams containing ketamine, amitriptyline and baclofen, the dose-finding most probably needs to start at 10% for ketamine and amitriptyline and 2.5% for baclofen, while the doses tested were much lower: 4%, 2% and 1% respectively. Topical analgesics are promising inroads for the treatment of neuropathic pain, once sufficient attention is given to aspects such as formulation and concentration.