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The natural lignan podophyllotoxin, a dimerized product of two phenylpropanoid moieties which occurs in a few plant species, is a pharmacologically important compound for its anticancer activities. It is used as a precursor for the chemical synthesis of the anticancer drugs etoposide, teniposide and etopophose. The availability of this lignan is becoming increasingly limited because of the scarce occurrence of its natural sources and also because synthetic approaches for its production are still commercially unacceptable. Biotechnological production using cell culture may be considered as an alternative source. Selection of the best performing cell line, its maintenance and stabilization are necessary prerequisites for its production in bioreactors and subsequent scale-up of the cultivation process to the industrial level. Scale-up of growth and product yield depends on a multitude of factors, such as growth medium, physicochemical conditions, seed inoculum, type of reactor and processing conditions. The composition of the growth medium, elicitors and precursors, etc. can markedly influence the production. Optimum levels of parameters that facilitate high growth and product response in cell suspensions of Podophyllum hexandrum have already been determined by statistical design. P. hexandrum cells have successfully been cultivated in a 3-l stirred-tank bioreactor under low shear conditions in batch and fed-batch modes of operation. The batch kinetic data were used to identify the mathematical model which was then used to develop nutrient-feeding strategies for fed-batch cultivation to prolong the productive log phase of cultivation. An improvement in the production of podophyllotoxin to 48.8 mg l(-1) in a cell culture of P. hexandrum was achieved, with a corresponding volumetric productivity of 0.80 mg l(-1) day(-1), when the reactor was operated in continuous cell-retention mode. Efforts are being made to further enhance its production levels by the development of hairy root culture or by varying the channeling of precursors towards the desired biosynthetic pathway by molecular approaches.

The effect of major medium ingredients (sugar, nitrogen source and phosphate) in Podophyllum hexandrum suspension cultures was investigated in order to increase the production of podophyllotoxin, the raw material in the synthesis of anticancer drugs. Amongst B5, Eriksson, MS, Nitsch, Street and White's medium, MS medium resulted in high growth and podophyllotoxin accumulation. The optimum level of nitrogen was found to be 60 mM, with a combination of ammonium salts and nitrate in the ratio of 1:2. The highest level of podophyllotoxin was obtained at 60 g glucose/l and at 1.25 mM phosphate after 30 days. Statistical design was adopted to determine the optimum levels of the parameters for cell growth and podophyllotoxin production.

Cytotoxic lignans derived from podophyllotoxin are currently used in cancer chemotherapy. Podophyllotoxin for semi-synthetic derivatization is isolated from the rhizomes of Podophyllum plants growing wild, some of which are counted as endangered species. An alternative source for podophyllotoxin or related lignans may in future be cell cultures derived from different plant species, such as Podophyllum spp or Linum spp. These cell cultures were shown to accumulate considerable amounts of podophyllotoxin or 5-methoxypodophyllotoxin. Optimization of the cell cultivation regime might lead to a renewable source of cytotoxic lignans for medicinal uses. This Mini-Review summarizes the attempts to establish plant cell cultures for the production of podophyllotoxin and related lignans and their optimization towards high levels of these target compounds. It also summarizes the results of studies on the biosynthesis of podophyllotoxin and 5-methoxypodophyllotoxin.