Antifungal Quinoline Alkaloids from <i>Waltheria indica</i>

Chemical investigation of a dichloromethane extract of the aerial parts of <i>Waltheria indica</i> led to the isolation and characterization of five polyhydroxymethoxyflavonoids, namely, oxyanin A (<b>1</b>), vitexicarpin (<b>3</b>), chrysosplenol E (<b>4</b>), flindulatin (<b>5</b>), 5-hydroxy-3,7,4′-trimethoxyflavone (<b>6</b>), and six quinolone alkaloids, waltheriones M–Q (<b>2</b>, <b>7</b>, <b>8</b>, <b>10</b>, <b>11</b>) and 5­(<i>R</i>)-vanessine (<b>9</b>). Among these, compounds <b>2</b>, <b>7</b>, <b>8</b>, <b>10</b>, and <b>11</b> have not yet been described in the literature. Their chemical structures were established by means of spectroscopic data interpretation including <sup>1</sup>H and <sup>13</sup>C, HSQC, HMBC, COSY, and NOESY NMR experiments and UV, IR, and HRESIMS. The absolute configurations of the compounds were established by ECD. The isolated constituents and 10 additional quinoline alkaloids previously isolated from the roots of the plant were evaluated for their in vitro antifungal activity against the human fungal pathogen <i>Candida albicans</i>, and 10 compounds (<b>7</b>, <b>9</b>, <b>11</b>–<b>16</b>, <b>18</b>, <b>21</b>) showed growth inhibitory activity on both planktonic cells and biofilms (MIC ≤ 32 μg/mL). Their spectrum of activity against other pathogenic <i>Candida</i> species and their cytotoxicity against human HeLa cells were also determined. In addition, the cytological effect of the antifungal isolated compounds on the ultrastructure of <i>C. albicans</i> was evaluated by transmission electron microscopy.