A compound of Formula I: or a pharmaceutically acceptable salt thereof, wherein: R 1 is selected from the group consisting of: C 1-4 alkyl, cyclopropylmethyl, cyclobutylmethyl and [1-(trifluoromethyl)cyclopropyl]methyl, each of which is optionally substituted with one to four substituents as allowed by valence independently selected from the group consisting of hydroxyl and F; R2 is hydrogen or methyl; when R 2 is hydrogen then R3 is selected from hydrogen, F or Cl; R4 is selected from hydrogen, F or Cl; R 5 is hydrogen; R6 is selected from hydrogen or F; and R 7 is selected from hydrogen, F or Cl; except that at least two of R 3, R 4, R 6 and R 7 must be F or Cl, unless R 3 is F in which case R 4, R 6 and R 7 may all be hydrogen; and if R 4 is Cl then R 7 cannot be Cl; when R 2 is methyl then R 3 is selected from hydrogen, methyl, F, Cl, or Br; R 4 is selected from hydrogen, methyl, F or Cl; R 5 is selected from hydrogen or F; R 6 is selected from hydrogen or F; and R 7 is selected from hydrogen, methyl, F or Cl; except that if R 5 is F then at least three of R 3, R 4, R 6 and R 7 must be F; and if R 4 is methyl or Cl then R 7 cannot be methyl or Cl; and R 8 is selected from the group consisting of: hydrogen, C 1-4 alkyl, cyclopropylmethyl, and cyclobutylmethyl, each of which is optionally substituted with one to three fluoro substituents as allowed by valence. R 1 is selected from the group consisting of: C 1-4 alkyl, cyclopropylmethyl, cyclobutylmethyl and [1-(trifluoromethyl)cyclopropyl]methyl, each of which is optionally substituted with one to four substituents as allowed by valence independently selected from the group consisting of hydroxyl and F; R2 is hydrogen or methyl; when R 2 is hydrogen then R3 is selected from hydrogen, F or Cl; R4 is selected from hydrogen, F or Cl; R 5 is hydrogen; R6 is selected from hydrogen or F; and R 7 is selected from hydrogen, F or Cl; except that at least two of R 3, R 4, R 6 and R 7 must be F or Cl, unless R 3 is F in which case R 4, R 6 and R 7 may all be hydrogen; and if R 4 is Cl then R 7 cannot be Cl; when R 2 is methyl then R 3 is selected from hydrogen, methyl, F, Cl, or Br; R 4 is selected from hydrogen, methyl, F or Cl; R 5 is selected from hydrogen or F; R 6 is selected from hydrogen or F; and R 7 is selected from hydrogen, methyl, F or Cl; except that if R 5 is F then at least three of R 3, R 4, R 6 and R 7 must be F; and if R 4 is methyl or Cl then R 7 cannot be methyl or Cl; and R3 is selected from hydrogen, F or Cl; R4 is selected from hydrogen, F or Cl; R 5 is hydrogen; R6 is selected from hydrogen or F; and R 7 is selected from hydrogen, F or Cl; except that at least two of R 3, R 4, R 6 and R 7 must be F or Cl, unless R 3 is F in which case R 4, R 6 and R 7 may all be hydrogen; and if R 4 is Cl then R 7 cannot be Cl; R 3 is selected from hydrogen, methyl, F, Cl, or Br; R 4 is selected from hydrogen, methyl, F or Cl; R 5 is selected from hydrogen or F; R 6 is selected from hydrogen or F; and R 7 is selected from hydrogen, methyl, F or Cl; except that if R 5 is F then at least three of R 3, R 4, R 6 and R 7 must be F; and if R 4 is methyl or Cl then R 7 cannot be methyl or Cl; and R 8 is selected from the group consisting of: hydrogen, C 1-4 alkyl, cyclopropylmethyl, and cyclobutylmethyl, each of which is optionally substituted with one to three fluoro substituents as allowed by valence.