That is, a drug molecule may preferentially be bound to either the inner or outer monolayer,

having to flip-flop in order to change the host monolayer. The typical flip-flop time can be large if the drug has some amphiphilicity or surface activity instead of being strongly lipophilic [40]. Drug molecules residing in the inner monolayer cannot be transported directly to another liposome; they first have to migrate to the outer monolayer. We denote by MdI and MdO the number of drug molecules residing in the inner (DI) and outer (DO) leaflets of donor liposomes, respectively. Similarly, MaI and MaO refer to the number of drug molecules residing Inhibitors,research,lifescience,medical in the inner (AI) and outer (AO) leaflets Inhibitors,research,lifescience,medical of acceptor liposomes. The reaction scheme in (10) can then be

generalized to account for the inter leaflet transport in donor and acceptor liposomes DI⇌K2dK1dDO  ⇌K2K1AO⇌K1aK2aAI. (23) Here, K1d and K2d are the two rate constants corresponding to the transfer of drugs between the two leaflets of the donor liposomes (and similarly for K1a and K2a referring to the acceptor liposomes). The rate constants K1 = (1 − kNd/M)KNa/N and K2 = (1 + kNa/M)KNd/N are identical to those for the single-state model, where K is given in (19). Based on (23), the rate equations Inhibitors,research,lifescience,medical can be written as M˙dO=KN(MaONd−MdONa+kNaNd)−K2dMdO+K1dMdI,M˙dI=K2dMdO−K1dMdI,M˙aO=KN(MdONa−MaONd−kNaNd)−K2aMaO+K1aMaI,M˙aI=K2aMaO−K1aMaI. (24) In the limit of a symmetric lipid bilayer, Inhibitors,research,lifescience,medical the two rate constants for flip-flop of a drug molecule from the inner to the outer leaf and from the outer to the inner leaf are identical (we note that the two leaflets of a liposomal bilayer are not strictly equivalent which, in a more refined model, would entail two different rate constants for flip-flop; this Inhibitors,research,lifescience,medical dependence on liposome curvature is neglected here). If we assume furthermore that donor and acceptor liposomes are chemically similar, we may write K1d = K2d = K1a = K2a = G as well as k = 0. In this case, the rate equations M˙dO=KN(MaONd−MdONa)−G(MdO−MdI),M˙dI=G(MdO−MdI),M˙aO=KN(MdONa−MaONd)−G(MaO−MaI),M˙aI=G(MaO−MaI) (25) depend on only two parameters,

the rate constants K and G. If we assume all drug molecules initially reside in the donor liposomes, the initial conditions are MdO(t = 0) = MdI(t = 0) = M/2, and MaO(t = 0) = MaI(t = 0) = 0, where M is the total number of drug molecules in the system. The solution of (25) can be http://www.selleckchem.com/btk.html expressed tuclazepam as MdI(t)=M2[NdN+NaN  ω2e−ω1t−ω1e−ω2tω2−ω1],MdO(t)−MdI(t)=M2KNaN  e−ω2t−e−ω1tω2−ω1,MaI(t)=MNa2N[1−ω2e−ω1t−ω1e−ω2tω2−ω1],MaO(t)−MaI(t)=M2KNaNe−ω1t−e−ω2tω2−ω1. (26) The solution is thus a combination of exponential decays with corresponding effective rate constants ω1 and ω2. Such biexponential behavior has been observed for the spontaneous transfer of certain lipids between phosphatidylcholine vesicles [41] and also for the release behavior of an imidazole derivate from liposomes [42].