# Polynitrogen Bicyclic and Tricyclic Compounds as PDE4 Inhibitors
## 3 Results and Discussion
### Chemistry
In contrast, **5b** (R4 = CONH(CH2)10 CH3) demonstrated a low activity (42% inhibition), somewhat in line with the result obtained with the long chain-based analogues of the first series (i.e., **3e-g**). In general, these data suggest that long and/or aliphatic chains reduce activity, either in R3 or R4, while short-chain polar terminal groups in both positions produce better inhibitory effects (see IC50 for **3b** and **5d**). As a result, **5d** demonstrates the most potent compound (IC50 = 3.54 uM) within these new products, supporting the evidence that the presence of NH(CH2)2 OH in R4 is highly beneficial for activity on PDE4. \begin{table}
\begin{tabular}{c c c c} \hline
**Comp.** & **R3** & **R4** & **IC50 (uM) a ** \\ \hline
3g &
\begin{tabular}{c} \(-\)NH(CH2)2-O- \\ (CH2)2 OH \\ \end{tabular} & H & 25\% \\ \hline
5a & NH2 & – \\  & &
\begin{tabular}{c} CONH(CH2)6 CH3 \\ \end{tabular} & 4.92 \(\pm\) 0.71 \\ \hline
5b & NH2 & – \\  & &
\begin{tabular}{c} CONH(CH2)10 CH3 \\ \end{tabular} & 42\% \\ \hline
5c & NH2 &
\begin{tabular}{c} \(-\)CONH(CH2)2-O- \\ (CH2)2 OH \\ \end{tabular} & 4.18 \(\pm\) 0.98 \\ \hline
5d & NH2 &
\begin{tabular}{c} \(-\)CONH(CH2)2 OH \\ \end{tabular} & 3.54 \(\pm\) 0.56 \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._ \begin{table}
\begin{tabular}{c c c c c} \hline
**Compd. 10** & **R\({}_{1}\)** & **R\({}_{6}\)** & **R\({}_{9}\)** &
\begin{tabular}{c} **IC\({}_{50}\) (nM) a** \\ **or \% Inhib. (1\(\mu\)M)** \\ \end{tabular} \\ \hline
**a** & \(-\)N(CH\({}_{2}\)CH\({}_{3}\))\({}_{2}\) & CH\({}_{3}\) & H & 62 \(\pm\) 5.3 \\ \hline
**b** & \(-\)N(CH\({}_{2}\)CH\({}_{3}\))\({}_{2}\) & H & H & NA b \\ \hline
**c** & \(-\)N(CH\({}_{2}\)CH\({}_{3}\))\({}_{2}\) & CH\({}_{3}\) & 3-furyl & 48\% \\ \hline
**d** & \(-\)N(CH\({}_{3}\))CH\({}_{2}\)Ph & CH\({}_{3}\) & H & 26\% \\ \hline
**e** & \(-\)N(CH\({}_{3}\))CH\({}_{2}\)Ph & H & H & 175.5 \(\pm\) 15.8 \\ \hline
**f** & \(-\)N(CH\({}_{3}\))C\({}_{3}\)H\({}_{7}\) & CH\({}_{3}\) & H & 17\% \\ \hline
**g** & \(-\)N(CH\({}_{3}\))C\({}_{4}\)H\({}_{9}\) & CH\({}_{3}\) & H & NA b \\ \hline
**i** & \(-\)N(CH\({}_{3}\))C\({}_{5}\)H\({}_{11}\) & CH\({}_{3}\) & H & NA b \\ \hline
**j** & \(-\)N(CH\({}_{3}\))C\({}_{2}\)H\({}_{5}\) & H & H & NA b \\ \hline
**k** & \(-\)N(CH\({}_{3}\))H\({}_{7}\) & H & H & 6\% \\ \hline
**l** & \(-\)N(CH\({}_{3}\))H\({}_{7}\) & H & H & NA b \\ \hline
**m** & \(-\)N(CH\({}_{3}\))H\({}_{7}\) & H & H & NA b \\ \hline
**n** & \(-\)N(CH\({}_{3}\))H\({}_{7}\) & H & H & NA b \\ \hline
**n** & \(-\)N(CH\({}_{3}\))H\({}_{7}\) & H & H & 5\% \\ \hline
**n** & \(-\)N(CH\({}_{3}\))H\({}_{7}\) & H & 4\% \\ \hline
**n** & \(-\)N(CH\({}_{3}\))H\({}_{7}\) & **0.84** \\ \hline \end{tabular}
\end{table} 
Table 5: PDE4 inhibitory activity of pyrazolo [1\({}^{\prime}\),5\({}^{\prime}\):1,6]pyrimidine [4,5-d]pyridazin-4(3\(H\))-ones **10a–q**. For the series of pyrazolo [1',5':1,6]pyrimido [4,5-\(d\)]pyridazin-4(3\(H\))-one derivatives (**10a-q**), our structure-activity relationships highlights that the structural variations at R\({}_{1}\) and R\({}_{6}\) also influence the inhibitory activity toward PDE4 significantly, as expressed by the IC\({}_{50}\) values (nM) or % inhibition at 1 \(\upmu\)M (Table 5). Preliminarily, it is worth noting that in position R\({}_{9}\), the hydrogen is present throughout the series, except in the case of **10c** (i.e., introduction of a 3-furyl group at R\({}_{9}\)). This latter possesses a moderate inhibition (48%), substantially lower than **10a** (i.e., the correspondent term, only differing for the presence of H at R\({}_{9}\), IC\({}_{50}\) = 62 nM), indicating that the introduction of the furyl heterocycle (and, perhaps, also other bulky, or aromatic groups) in R\({}_{9}\) may interfere with the binding. Therefore, the H has been kept constant in R\({}_{9}\) for the design of this library of tricyclic analogues, in order not to hinder optimal interactions with PDE4, thereby favoring activity.