Delivery and retention of active agents within the skin

A system comprising a penetrating agent having a structure of formula (I-a):

• or pharmaceutically acceptable salt thereof,
• wherein
• A is or comprises a glycosaminoglycan;
• L is selected from an optionally substituted C1-6 alkylene chain wherein one, two, or three methylene units of L are optionally and independently replaced by —NH—, —O—, —S— or —C(O)—, —NH—, —O—, —S—, —S(O)—, —S(O)2—, —C(O)—, polyethylene glycol, and a moiety which results from a click reaction, nucleophilic addition, or Michael addition;
• R1 is independently —H, halogen, —CN, optionally substituted C1-6 aliphatic, optionally substituted 5- to 10-membered heterocyclyl, optionally substituted 6- to 10-membered aryl, or optionally substituted 5- to 10-membered heteroaryl;
• R2 is —H, halogen, —CN, optionally substituted C1-6 aliphatic, optionally substituted 5- to 10-membered heterocyclyl, optionally substituted 6- to 10-membered aryl, or optionally substituted 5- to 10-membered heteroaryl;
• and
• m is 1-3.