SLC6A3 has also been the analyzed for its role in modulating therapeutic responses to sub-chronic (8 weeks) administration of methylphenidate. Similar to the Rohde et al. (2003) study discussed above, Cheon et al. (2005) used SPECT to measure dopamine transporter density in several brain regions (basal ganglia, right basal ganglia, and occipital cortex) of 11 children diagnosed with ADHD treated with varying doses of methylphenidate, and measured both clinical response and transporter density. Children with at least one copy of the 9-repeat allele (n = 4) showed a better therapeutic response to methylphenidate than 10/10 individuals (4 out of 4 responders versus 2 out of 7 responders in the 10/10 group), and significantly lower dopamine transporter density than 10/10 individuals. This suggests that 10/10 individuals may require more methylphenidate due to increased dopamine transporter density within the brain.
