The ornamental design for an interdentalbrush, as shown and described.
FIG. 1 is a perspective view of an interdental brush showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a side view thereof;
FIG. 4 is an opposite side view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a top view thereof.
The uneven-length broken lines represent unclaimed boundaries of the claimed design. The even-length broken lines showing the remainder of the interdental brush represent unclaimed subject matter and form no part of the claimed design.