PET Imaging of Distinct Brain Uptake of a Nanobody and Similarly-sized PAMAM Dendrimers After Intra-arterial Administration

Introduction: We have recently shown that intracerebral delivery of an anti-VEGF monoclonal antibody bevacizumab using an intra-arterial (IA) infusion is more effective than intravenous administration. While antibodies are quickly emerging as therapeutics, their disadvantages such as large size, production logistics and immunogenicity motivate search for alternatives. Thus we have studied brain uptake of nanobodies and polyamidoamine (PAMAM) dendrimers.

Methods: Nanobodies were conjugated with deferoxamine (DFO) to generate NB(DFO). Generation-4 PAMAM dendrimers were conjugated with DFO, and subsequently primary amines were capped with butane-1,2-diol functionalities to generate G4(DFO)(Bdiol). Resulting conjugates were radiolabeled with zirconium-89. Brain uptake of ZrNB(DFO) and ZrG4(DFO)(Bdiol) upon carotid artery vs tail vein infusions with intact BBB or osmotic blood-brain barrier opening (OBBBO) with mannitol in mice was monitored by dynamic positron emission tomography (PET) over 30 min to assess brain uptake and clearance, followed by whole-body PET-CT (computed tomography) imaging at 1 h and 24 h post-infusion (pi). Imaging results were subsequently validated by ex-vivo biodistribution.

Results: Intravenous administration of ZrNB(DFO) and ZrG4(DFO)(Bdiol) resulted in their negligible brain accumulation regardless of BBB status and timing of OBBBO. Intra-arterial (IA) administration of ZrNB(DFO) dramatically increased its brain uptake, which was further potentiated with prior OBBBO. Half of the initial brain uptake was retained after 24 h. In contrast, IA infusion of ZrG4(DFO)(Bdiol) resulted in poor initial accumulation in the brain, with complete clearance within 1 h of administration. Ex-vivo biodistribution results reflected those on PET-CT.

Conclusions: IA delivery of nanobodies might be an attractive therapeutic platform for CNS disorders where prolonged intracranial retention is necessary.