consumption. inhibition of hif-1, which is one of the main effects of kalkitoxin, thus induces cellular hypoxia. toxicity kalkitoxin is ichthyotoxic to goldfish (carassius auratus, lc50: 700nm) and to aquatic crustacean brine shrimp (artemia salina, lc50: 150-180nm ). kalkitoxin also has been shown to have delayed neurotoxic effects on cerebellar granule cells of the rat (lc50: 3,86nm). therapeutic research many efforts to discover cancer therapeutic drugs focus on the screening of novel biomolecules produced and isolated from various plants and animals. these isolated molecules are screened via in-vitro assays to measure their effects in standardized paradigms designed to select for the desired therapeutic effect. kalkitoxin was originally isolated from lyngbya majuscula as an effort to collect new molecules for testing as antitumor or antifungal agents. one of the first tests of kalkitoxin tumor-selective cytotoxicity used an in-vitro assay to test solid tumor selectivity of kalkitoxin's previously demonstrated cytotoxicity against the human colon cell line hct-116. the assay measured the extent of differential cytotoxicity of kalkitoxin and various analogous structures by observing differential cytotoxicity against solid tumor cells, and either non-solid tumor cells such as leukemia cells or normal cells. this test yielded promising results, as kalkitoxin exhibited preferential cytotoxicity for the solid tumor cell test conditions (colon 38, and hct-116 cells) as compared to the non-solid tumor and normal cell conditions. kalkitoxin exerts this cytotoxic effect through inhibition of the mitochondrial electron transport chain complex 1. this causes the inhibition of hypoxia-induced hif-1 activation, which is crucial in solid tumor cancers because hypoxia drives tumor angiogenesis, leading to worsening disease stages and increased resistance to treatment. hif-1 is a transcription factor which induces expression of genes promoting oxygen availability and decreasing oxygen consumption, the effect of which counteracts cellular hypoxia. therefore, kalkitoxin's hif-1 inhibitory ability positions it as a potentially promising molecule to counteract the progression of some solid tumor cancers by blocking the tumor proliferative response to hypoxia. the caveat to kalkitoxin's promising anti-proliferative properties is its neurotoxic effects. at concentrations comparable to those required for tumor-selective cytotoxicity, kalkitoxin induces cell death when applied to rat cerebellar granule neurons (cgn) in culture. kalkitoxin acts as an n-methyl-d-aspartate (nmda) receptor agonist, and induces cytotoxicity in cultured rat cgns at delayed time points. therefore, this effect must be taken into account when considering kalkitoxin or its chemical derivatives for use as a therapeutic option.