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Synthesis and evaluation of an indexed combinatorial library for lead optimization

John W. Lyga,* Syed F. Ali, Charles A. Webster, Kirk A. Simmons, Joan Plummer and Malgorzata E. Galietti

Discovery Research Department, Agricultural Products Group, FMC Corporation, PO Box 8, Princeton, New Jersey 08543, USA
Compound 1 was identified as a weak lead from our general screen having activity against mosquito (A. aegypti, pLC50 = 5.8). Although no references to insecticidal activity were found in a literature search, this area was evaluated in the 1970s for antiinflammatory agents, CNS depressants and as antifungal agents.[1,2]

Compound 1

We decided to optimize both the N-methyl groups simultaneously using a factorial design strategy, representing pi (lipophilicity), sigma (electronics), and L (length). This required a total of 9 X 9 or 81 compounds to test every possible combination. Since the synthesis of this class had been worked out, it seemed to us to be an ideal candidate for optimizing activity using an "indexed" combinatorial library [3,4].

To prepare the 81 compound library, a total of 18 compounds were required as starting materials; a set of nine pyrazoles (pyrazole prelibrary) and a set of nine piperidones (piperidone prelibrary).

Pyrazole pre-library

pyrazole pre-library
X = CH(CH3)2, CH2C(CH3)3, CH2CF3, CH2CH2CN,
CH2CH2OCH3, CH2C6H5, CH3, C6H5, CH2CH2CH2CH3

Piperidone pre-library

piperadone pre-library
Y = COCH2CH3, COC6H5, CH(CH3)2, CH2CH2F, CH2CH2OCH3,
CH2CN, CH3, CO2CH(CH3)2, CH2CH2CH2CH2CH3

Each prelibrary was pooled and divided into nine separate flasks and each reacted with the appropriate reagents as indicated in Figure 1 (piperidone pool) and Figure 2 (pyrazole pool) to generate the following mixtures:

Best Structures
mixture table

Due to synthesis problems, Mixture number 3 was not tested in the screen. The remaining 17 mixtures were tested in our mosquito assay and the results are shown in Table 1 or in 3D graphical representation. The more active mixtures were mixture's number 7, 12-14.

Data matrix for mixtures on mosquito

Red = data for mixtures (relative potency, 4=most active)

Blue = pure compounds (pLC50)

data matrix (3731bytes)

To confirm the results obtained from the relative potency data, the three compounds predicted to be the more active were synthesized independently along with the three theoretical "best" from mixture number 3. In addition, we prepared the compound predicted to be the least active, having R = butyl (mixture number 8) and R' = isopropylcarboxy (mixture number 17).

Results and Discussion

The three compounds synthesized from mixture number 7 were found to be equivalent in activity to compound 1, the lead. The three compounds from mixture number 3 were on average less active than those from mixture number 7, although one (R = CH2CF3, R' = CH2CH2F) was close in activity. The predicted least active compound, the intersection of mixture numbers 8 and 17, was confirmed as inactive.

The major advantage of using an "indexed" combinatorial library is to improve synthesis efficiency; not having to synthesize every compound of interest independently. A limitation, however, is the requirement of relying on ranking activity data of mixtures. As we found in this study, if the absolute range in activity for the mixtures is small, it becomes increasingly difficult to differentiate between mixtures that are close inactivity, such as a rank order of 3 versus 4 due to the variance in the assay data (± 0.3 pLC50 ).

Conclusion

A total of 27 compounds were synthesized or resynthesized from the total 81, including the three compounds needed to characterize Mixture number 3. Although none of the compounds were found to be more potent than the lead, the "indexed" combinatorial library approach allowed us to reach this conclusion without the need to synthesize every compound independently.

Acknowledgement

The authors are indebted to Dr Jonathan S. Baum for converting this poster to hypertext markup language.