\begin{tikzpicture}[
    mysquare/.style={
    rectangle,
    draw=black,
    fill=white,
    fill opacity = 0.3,
    text opacity=1,
    inner sep=0pt,
    minimum size=40pt,
    font=\small,
    align=center},
    myarrow/.style={-Stealth},
    node distance=0.6cm and 1.0cm
  ]

  \node[mysquare] (c1) {Prol};
  \node[mysquare,draw=white,minimum size=20pt,below left= of c1] (e) {$E_{drug}$};
  \node[mysquare, minimum size=30pt,right=of c1] (t1) {Transit\\1};
  \node[mysquare, minimum size=30pt, right=of t1] (t2) {Transit\\2};
  \node[mysquare, minimum size=30pt, right=of t2] (t3) {Transit\\3};
  \node[mysquare, right=of t3] (c2) {Circ};
  \node[mysquare,draw=white,below= of c2] (k) {};
  \node[mysquare,draw=white,minimum size=20pt,font=\small,above left= of c1] (kp) {$k_{prol}(=k_{tr})$};

    \foreach \i/\j/\txt/\p/\r in {% start node/end node/text/position/rotation
      c1/t1/$k_{tr}$/above/0,
      t1/t2/$k_{tr}$/above/0,
      t2/t3/$k_{tr}$/above/0,
      t3/c2/$k_{tr}$/above/0,
      c2/k/$k_{circ}(=k_{tr})$/right/90
    }
    \draw [myarrow] (\i) -- node[sloped,font=\small,\p,rotate=\r] {\txt} (\j);

  \draw [-Stealth] (c2) edge[in=85,out=90,looseness=0.5] node[above]{Feedback=$\left( \frac{Circ_0}{Circ} \right)^\gamma$}(c1);

  \path (c2) edge[in=315,out=225,looseness=0.5] node[below]{MTT}(c1);

  \draw [-Stealth] (c1) edge [out=95,in=150,looseness=8] node[above left] {} (c1);
  \draw [-Stealth] (e) edge [out=90,in=270] node[left] {} (kp);

\end{tikzpicture}
