Distribution, Metabolism and Tumoricidal Activity of Doxorubicin Administered in Sorbitan Monostearate (Span 60) Niosomes in the Mouse

Abstract

Purpose. Encapsulation of doxorubicin in niosomes was sought as a route to tumour targeting and improved tumoricidal through the alteration of doxorubicin pharmacokinetics and metabolism.

Methods. Doxorubicin niosomes (10 mg kg^−l doxorubicin) prepared from sorbitan monostearate (Span 60), cholesterol and choleth-24 (a 24 oxyethylene cholesteryl ether) in the molar ratio 45:45:10 were administered intravenously to female NMRI mice bearing the MAC 15A subcutaneously implanted tumour. Plasma doxorubicin was fractionated by gel filtration and quantified by HPLC with fluorometric detection as niosome-associated doxorubicin and released doxorubicin. Tumoricidal activity of the formulation was assessed by the intravenous injection of 5 mg kg⁻¹ and 10 mg kg⁻¹ doxorubicin niosomes to male NMRI mice bearing a 6 day old MAC 15A tumour.

Results. At least 90% of the plasma doxorubicin was associated with the niosome fraction 4 h after dosing, and 50% was still associated after 24 h. The clearance of doxorubicin released from the niosomes was about 10 fold greater than the clearance of niosomal doxorubicin (176.5 mL h^−l and 16.2 mL h⁻¹, respectively). The area under the plasma level-time curve increased 6 fold when doxorubicin was administered in niosomes, compared to doxorubicin solution (66.0 µg.h mL^−l and 10.3 µg.h mL⁻¹, respectively). The area under the tumour level time curve was increased by over 50% by the administration of doxorubicin in niosomes when compared to the drug administered in solution (58.6 µg.h mL^−l and 34.3 µg.h mL⁻¹, respectively). There was no statistically significant difference between levels of the drug in the heart when niosomal doxorubicin or doxorubicin solution were administered. Doxorubicin metabolites, namely doxorubicinol and the aglycones doxorubicinone, doxorubicinolone and 7-deoxydoxorubicinone, were found associated with the niosomes in the plasma, possibly due to their adsorption to the vesicle surface once formed outside the niosome. Overall metabolite levels in the liver were increased when doxorubicin niosomes were administered compared to the drug in solution. A 5 mg kg⁻¹ injection of doxorubicin niosomes produced a terminal mean tumour weight that was similar to that obtained from animals administered 10 mg kg⁻¹ doxorubicin solution.

Conclusions. Modest tumour targeting was achieved by the delivery of doxorubicin in sorbitan monostearate niosomes, increasing the tumour to heart AUC^0–24 ratio from 0.27 to 0.36 and a doubling of tumoricidal activity. The overall level of doxorubicin metabolites was also increased.