Crucially, XL01126, showed around 3-fold more potently inhibited Rab10 phosphorylation in WT MEFs than HG-10-102-01 (Figure ), and 6-fold more potently inhibited Rab10 phosphorylation in G2019S LRRK2 MEFs (Figure ). These observations suggest that converting HG-10-102-01 to a PROTAC degrader not only improves downstream signaling inhibition, but also increases selectivity for G2019S LRRK2 over WT. Cis-XL01126 (Scheme 1), a non-degrading distomer control of XL01126 where the stereochemistry at the hydroxyl group of hydroxyproline is inverted to abrogate VHL binding , showed no degradation of WT LRRK2 (Figure ) and G2019S LRRK2 (Figure ), but inhibited Rab10 phosphorylation at a similar potency as HG-10-102-01 in both WT MEFs (Figure ) and G2019S LRRK2 MEFs (Figure ). However, due to the lack of LRRK2 degradation, cis-XL01126 was around 7-fold less potent than XL01126 in inhibiting Rab10 phosphorylation (116 nM vs 15 nM), further demonstrating the potency boost in downstream functionality achieved from LRRK2 degradation over and above kinase inhibition. Figure . XL01126 degrades LRRK2 in human peripheral blood mononuclear cells (PBMCs) derived from healthy donors, and mouse bone marrow derived macrophages (BMDMs). Representative Western blotting of total LRRK2, LRRK2-pSer935, pRab10, Rab10 total, and GAPDH levels following treating the PBMCs with XL01126 and cis-XL01126 at the indicated concentrations for 4h (A) and 24 h (B). The relative LRRK2 protein and pRab10 levels were obtained by quantifying the ratios of total LRRK2/GAPDH or Rab10-pThr73/total Rab10, respectively, and the ratios were normalized to the DMSO treated samples. The relative LRRK2 and pRab10 protein levels were plotted against the compounds concentration and fitted against "non-linear regression, one site-fit LogIC50" in GraphPad to obtain the DC50 and EC50 values. Data points are presented as mean ± SEM from three biological independent replicates. (C) Representative Western blotting of total LRRK2, LRRK2-pSer935, pRab10, Rab10 total, and GAPDH levels following treating the PBMCs with 300 nM of XL01126 and cis-XL01126 for the indicated time periods. The relative LRRK2 protein and pRab10 levels were obtained by quantifying the ratios of total LRRK2/ GAPDH or Rab10-pThr73/total Rab10, respectively, and the ratios were normalized to the DMSO treated samples. The relative LRRK2 and pRab10 protein levels were plotted against the treatment time and were fitted against "non-linear regression, one phase decay" in GraphPad to obtain the half-life (T1/2) values. Data points are presented as mean ± SEM from three biological independent replicates. (D) Representative Western blotting of LRRK2 total and Tubulin levels after treating BMDMs with XL01126 and cis-XL01126 for 4h. The relative LRRK2 levels were obtained by quantifying the ratios of total LRRK2/Tubulin, and normalized to the DMSO treated samples. The relative LRRK2 levels were plotted against the compounds concentration and fitted against "non-linear regression, one site-fit LogIC50" in GraphPad to obtain the DC50 values.