  Flavored mesons containing quarks of unequal masses are studied. The
appropriate tool is the Bethe-Salpeter formalism, but its inherent complexity
leads to series of difficulties mostly related to the central role played in it
by the relative time or energy. We consider bound states in the spirit of
"Constraint Relativistic Quantum Mechanics (RQM)". Interaction of quarks is
described by the funnel-type potential with the distant dependent strong
coupling, $\alpha_s(r)$. Relativistic bound-state problem is formulated with
the use of symmetries, energy-momentum conservation laws in Minkowskiy space.
Relativistic two-body wave equation with position dependent particle masses is
derived and used to describe the flavored mesons. Free particle hypothesis for
the bound state is developed: quark and antiquark move as free particles in of
the bound system. Solution of the equation for the system in the form of
a~standing wave is given. Interpolating complex-mass formula for two exact
asymptotic eigenmass expressions is obtained. Mass spectra for some
leading-state flavored mesons are calculated.
