Treatment with PF-360, an investigational leucine-rich repeat kinase 2 (LRRK2) inhibitor, can efficiently decrease LRRK2’s phosphorylation levels, known to be elevated in Parkinson’s patients, in the brains of a mouse model of Parkinson’s disease, a preclinical study reports.
However, despite some observed dose-dependent therapeutic effects, including gait improvement, no robust changes in dopaminergic function were observed.
Results of the study were recently presented during the Society for Neuroscience’s 2018 conference in San Diego in a poster titled “Assessment of the Anti-parkinsonian Effects of the Potent and Selective LRRK2 Kinase Inhibitor PF-360 in the AAV-A53T Mouse Model of Parkinson’s Disease.”
The study was the result of a collaboration between several institutions including Charles River Discovery, Merck, Pfizer, Atuka Inc., and The Michael J. Fox Foundation for Parkinson’s Research.
The LRRK2 gene provides instructions for making a kinase, which is a protein that regulates the function of other molecules. Mutations in this gene put the protein into an overly activated state.
Mutations in the LRRK2 gene are one of the most commonly known genetic causes of Parkinson’s disease and usually result in the malfunctioning of lysosomes — special compartments within cells that digest and recycle different types of molecules. Lysosomal dysfunction is involved in the formation of Lewy body protein aggregates and, therefore, neurodegeneration.
Scientists believe that blocking LRRK2’s activity has the potential to slow disease progression.
Using a selective LRRK2 inhibitor called PF-360, researchers studied the dose-response efficacy of the potential therapy in two different mouse models (C57BL/6J and LRRK2-G2019S) that were injected with a “biological cocktail” of an adeno-associated virus combined with a human mutated A53T alpha-synuclein (AAV-A53T) — the major component of protein clumps called Lewy bodies, a hallmark of Parkinson’s.
They used 90 C57BL/6J mice 10-12 weeks old and 105 LRRK2-G2019S mice, 75 of which were 11-12 weeks old and 30 were 5-6 months old. In mouse “time,” 12 weeks is equal to adulthood.
This induced the degeneration of dopaminergic neurons in an area of the brain called the substantia nigra and decreased dopamine and tyrosine hydroxylase — the enzyme responsible for catalyzing levels of L-DOPA, the precursor to dopamine — in the striatum, mimicking Parkinson’s disease.
Mice were treated for 42 days with a diet containing PF-360 or a placebo (control), which was begun seven days prior to AAV-A53T injections.
PF-360 inhibited LRRK2 phosphorylation in the animals’ brain cortex and lungs at a specific site of the protein called serine 935 (serine is an amino acid, or the proteins’ building block). This protein region is required for interaction of LRRK2 with other molecules.
Phosphorylation (the adding of a phosphate group) alters a protein’s structure turning it, for instance, into an activated or deactivated state. As such, phosphorylation is the most common mechanism of regulating protein function and transmitting signals throughout the cell.
Pronounced therapeutic effects were observed with increasing doses (1 mg/kg, 3 mg/kg, 10 mg/kg, 30 mg/kg, and 60 mg/kg of PF-360) in both animal strains and age groups.
AAV-A53T injection led to motor impairments such as decreased speed (longer stride duration, shorter step length), slower swing speed, and reduced hind limb protraction (forward extension).
LRRK2-G2019S mice at 11-12 weeks old recovered their hind limb protraction and retraction with 10 mg/kg of PF-360, while older animals at 5-6 weeks of age had their overall speed (stride duration and swing speeds) improved with 30 mg/kg of the treatment.
No gait changes were observed after 42 days of PF-360 treatment in C57BL/6J mice. However, there was an insignificant treatment-related trend toward increased tyrosine hydroxylase-positive cells in the substantia nigra of C57BL/6J animals.
After treatment, a significantly higher number of tyrosine hydroxylase-positive cells were observed in older LRRK2-G2019S mice.
An increase in tyrosine hydroxylase-positive cells is indicative of an increase in the number of nerve cells that can produce either L-DOPA or dopamine.
Neurochemical analysis revealed that PF-360 delivery to younger animals did not improve striatum levels of dopamine or the intermediate end products of dopamine’s metabolism (3,4-dihydroxyphenylacetic acid and homovanillic acid).
However, treatment significantly increased homovanillic acid levels in older LRRK2-G2019S mice.
Given that most evidence suggests an LRRK2 contribution to Parkinson’s disease via abnormal phosphorylation, this study shows that although PF-360 can reduce LRRK2 phosphorylation levels, both in the brain and in the periphery, it failed to show robust improvements in dopaminergic function.