PLX5622 hemifumarate is a highly selective brain penetrant and orally active CSF1R inhibitor with an IC 50 of 0.016 μM and Ki of 5.9 nM. PLX5622 hemifumarate allows for extended and specific microglial elimination, preceding and during pathology development. In vivo, PLX5622 hemifumarate demonstrates desirable PK properties in varies animals [1] [2].

PLX5622 (1-20 μM; 3 days) hemifumarate effectively depletes microglia while not affecting oligodendrocytes or astrocytes in cerebellar slices. PLX5622 (4 μM; 3 days) hemifumarate leads to a 30-40% reduction in NG2+ or PDGFRα+ cells, increasing to 90-95% at 20 μM. No reduction of NG2+ or PDGFRα+ OPCs is observed in slices exposed to 1 μM or 2 μM PLX5622 despite robust (~95%) depletion of the microglial cells [3].

Pharmacodynamics of PLX5622 hemifumarate in preclinical studies PLX5622 (1200 ppm; chow; for 3 weeks or 3 days; adult C57/Bl6 wild type mice) hemifumarate causes around 80% of microglia lost after 3 days of treatment and a 99% microglia loss after 3 weeks of treatment. PLX5622 (adult C57/Bl6 wild type mice aged 3 months; diet for 3 weeks) reduces microglia in cortex, striatum, cerebellum and hippocampus [4]. PLX5622 (50 mg/kg; intraperitoneal injection; once (neonatal rat) or twice (adult rat) a day; for a total of 14 days) hemifumarate depletes microglia by 80-90% within 3 days of treatment, which increases to > 90% by 7 days. After 14 days of PLX5622 treatment, microglia is depleted by > 96% in both neonates and adults while preserving baseline astrocyte quantity. (A single daily injection of 0.65% PLX5622 suspended in 5% dimethyl sulfoxide and 20% Kolliphor RH40 in 0.01 M PBS is sufficient for neonatal microglia depletion, adult depletion requires injections twice daily) [5]. PLX5622 (formulated in AIN-76A standard chow at 1200 mg/kg; for 28 days) hemifumarate leads to reduction in microglia throughout the CNS in 14-month-old 5xfAD mice [6]. Pharmacokinetics of PLX5622 hemifumarate in preclinical species [4] Species IV PO (gavage) Dose (mg/kg) AUC 0-∞ (ng hr/mL) CL (mL/min/kg) Vss (L/kg) t 1/2 (hr) Dose (mg/kg) AUC 0-∞ (ng hr/mL) Cmax (ng/mL) F Mouse 1.92 15,500 2.1 0.34 2.6 45 215,000 26,300 59% Rat (male) 1.13 2,630 7.7 1.2 2.3 45 99,600 12,000 95% Rat (female) 1.13 5,110 3.7 1.0 3.9 45 181,000 15,600 89% Dog 1.00 6,230 3.0 2.3 15 45 96,500 3,630 34% Monkey 1.35 2,100 11 1.6 2.2 ND ND ND ND Preparation of gavage dosing suspensions for PLX5622 hemifumarate [4] PLX5622 hemifumarate is dissolved in DMSO at a concentration that is 20x the final dosing solution. The compound stock is protected from light. A fresh stock is made each week. The components of the diluent generally are prepared a day or more in advance because they take time to dissolve completely: a) 2% hydroxypropyl methyl cellulose (HPMC): 2.0 g powder was brought to 100 mL deionized water; b) 25% Polysorbate 80 (PS80): 25 g was brought to 100 mL deionized water. To make 100 mL diluent, add 25 mL of 2% HPMC stock (0.5% final) and 4 mL of 25% PS80 stock (1% final) to 71 mL deionized water to have final 100 mL. Final composition after mixing with compound: 0.5% HPMC, 1% PS80, 5% DMSO. On each dosing day, the compound stock is diluted 20-fold as follows: 19 volumes of diluent are measured into the tube, and 1 volume of the 20x compound/DMSO stock is added. The cap is closed and the content of the tube is mixed by inversion and placed in a sonicating water bath to make a uniform suspension.