AutoCAD .NET :: Bind MLeader To An Entity

I have a method DrawMLeaderLine(Database db, Transaction tr, Point3d point, string text) that draws me a MLeader line to the given point with the given text as annotation. The MLeader line points to the midpoint of a Line entity. What I need is a binding between the Line midpoint and the MLeader line, so when I move or resize the Line, the MLeader line should follow! I already tried to solve that, using constraints, but it doesn't work for Leaders.

This is my code...
 
public static MLeader DrawMLeaderLine(Database db, Transaction tr, Point3d point, string text) { Point3d mTextLocation = point + new Vector3d(5, 5, 0); BlockTableRecord btr = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) as BlockTableRecord; MText mText = new MText(); mText.SetDatabaseDefaults(); mText.SetContentsRtf(text); [code]........ 

will be called from a different class...
 
Line line = DrawingUtil.DrawLine(doc.Database, tr, closestPoints.Item1, closestPoints.Item2);  //Get midpoint of line            double midPointX = (line.EndPoint.X + line.StartPoint.X) / 2.0d;            double midPointY = (line.EndPoint.Y + line.StartPoint.Y) / 2.0d;            Point3d midPointOfLine = new Point3d(midPointX, midPointY, 0);  //Draw the leaderline            MLeader leaderLine = DrawingUtil.DrawMLeaderLine(doc.Database, tr, midPointOfLine, "Connection");