Biopharmaceuticals from Plants: A Multitude of Options for Posttranslational Modifications

Transient Expression of Human Cytochrome P450s 2D6 and 3A4 in Nicotiana benthamiana Provides a Possibility for Rapid Substrate Testing and Production of Novel Compounds

Employment of transient expression of foreign genes for bioconversion of pharmaceutically valuable low-molecular-weight compounds, including plant secondary metabolites, is an enticing trend still scantily explored in plant biotechnology. In the present work, an efficient protocol for rapid assessment of synthetic and plant-derived metabolites as potential substrates for human P450s (CYP2D6 and CYP3A4) via Agrobacterium-mediated transient expression in Nicotiana benthamiana is put forth. Animal P450s with broad substrate specificity are promising candidates for transformation of diverse metabolites. The efficiency of P450s in heterologous surroundings is not always satisfactory and depends on the availability of an associated electron-transfer enzyme. Plants represent an attractive assortment of prospective hosts for foreign P450s expression. The optimal composition of genetic blocks providing the highest transient expression efficiency is designed, an effective substrate administration scheme is validated, and biological activity of the investigated P450s against loratadine and several indole alkaloids with different molecular scaffold structures is tested. A novel indole alkaloid, 11-hydroxycorynanthine, is isolated from N. benthamiana plants transiently expressing CYP2D6 and supplemented with corynanthine, and its structure was elucidated. The proposed technique might be of value in realization of combinatorial biosynthesis concept comprising the junction of heterologous enzymes and substrates in different metabolic surroundings.

Predictive models for transient protein expression in tobacco (Nicotiana tabacum L.) can optimize process time, yield, and downstream costs

The transient expression of recombinant biopharmaceutical proteins in plants can suffer inter-batch variation, which is considered a major drawback under the strict regulatory demands imposed by current good manufacturing practice (cGMP). However, we have achieved transient expression of the monoclonal antibody 2G12 and the fluorescent marker protein DsRed in tobacco leaves with ∼ 15% intra-batch coefficients of variation, which is within the range reported for transgenic plants. We developed models for the transient expression of both proteins that predicted quantitative expression levels based on five parameters: The OD(600 nm) of Agrobacterium tumefaciens (from 0.13 to 2.00), post-inoculation incubation temperature (15-30°C), plant age (harvest at 40 or 47 days after seeding), leaf age, and position within the leaf. The expression models were combined with a model of plant biomass distribution and extraction, generating a yield model for each target protein that could predict the amount of protein in specific leaf parts, individual leaves, groups of leaves, and whole plants. When the yield model was combined with a cost function for the production process, we were able to perform calculations to optimize process time, yield, or downstream costs. We illustrate this procedure by transferring the cost function from a production process using transgenic plants to a hypothetical process for the transient expression of 2G12. Our models allow the economic evaluation of new plant-based production processes and provide greater insight into the parameters that affect transient protein expression in plants.