Assessment of the potential uses of liposomes for lymphoscintigraphy and lymphatic drug delivery failure of 99 m-technetium marker to represent intact liposomes in lymph nodes

Abstract

The in vivo fate of subcutaneously injected neutral SUV liposomes in rats was examined using a membrane marker, 99 m Tc, and an aqueous marker, 125I-labelled poly(vinyl pyrrolidone). Liposomes with entrapped 125I-labelled poly(vinyl pyrrolidone) were labelled with 99 m Tc by the SnCl 2 method [2]. 99 m Tc-radioactivity was localized several-fold more in the primary and secondary regional lymph nodes than 125I-labelled poly(vinyl pyrrolidone)-radioactivity. Similarly, 99 m Tc-radioactivity appeared and was subsequently cleared from the circulation much more rapidly than 125I-labelled poly(vinyl pyrrolidone). The gel chromatography of the lymph node homogenate revealed that 60–70% of 125I-labelled poly(vinyl pyrrolidone)-radioactivity was in the liposome fractions, whereas only 3% of 99 m Tc-radioactivity was co-eluted with liposomes. Thus, the two markers have different fates in the lymphatics, and the presence of all 99 m Tc-radioactivity does not represent the 60–70% of intact liposomes present in lymph nodes. Using the aqueous marker 125I-labelled poly(vinyl pyrrolidone), the lymphnode localization of positive, negative and neutral small unilamellar vesicles was studied, and it was found that 125I-radioactivity was more localized from negative liposomes than from positive liposomes, which in turn was more localized than that from neutral liposomes. Thus, these findings differ from those reported earlier [2], where the authors used 99 m Tc as a liposomal marker. In vitro studies showed that liposomes of preparations containing 20 mol% cholesterol became ‘leaky’ to low-molecular-weight drugs, for example, methotrexate ( M r 454) to a much greater extent than with a large-molecular-weight substance, 125I-labelled poly(vinyl pyrrolidone) ( M r 30 000–40 000), when incubated with rat lymph at 37°C. Using the two markers 99 m Tc and 125I-labelled poly(vinyl pyrrolidone) it was found that the localization of both radioactivities was reduced in lymph nodes draining λ-carrageenan-treated footpads. In conclusion, it is suggested that liposomes can be used for the delivery of drugs to diseased lymph nodes, and it would be worthwhile examining the possibilities of using alternative methods of labelling liposomes with 99 m Tc rather than using the SnCl 2 technique [2], or using other radionuclides as markers for γ-scan imaging.