Monitoring of intracellular ribonucleotide pools is a powerful tool in the development and characterization of mammalian cell culture processes.

Efficient cell culture process development for the industrial production of recombinant therapeutics is characterized by constraints which pertain to issues such as costs, competitiveness and the meeting of project timelines. These constraints require tools which can help the developer learn as much as possible as quickly as possible about the cell at hand and identify features of a particular culture which are amenable to improvement. Current on- and off-line monitoring parameters, however useful, provide only late indications (cell concentration, viability) and circumstantial evidence (lactate, ammonia, etc.) with regard to the physiologic status of cells at the time of sampling. The relative intracellular content of purine to pyrimidine nucleotide triphosphates as well as the ratio of UTP to UDP-N-acetylhexosamines have been previously described as sensitive indicators of a cell's metabolic status, growth potential, and overall physiological condition. The sensitivity of such nucleotide ratios and their usefulness in commercially relevant process development and characterization were tested at Boehringer Ingelheim Pharma KG in a large number of fermentations (>80) with a variety of culture modes, cells, and products in scales up to 10,000 litres. Monitoring of these intracellular parameters allows a timely and reliable assessment of cell state and growth potential, which is possible neither by classical cell number and viability measurements nor by a variety of fermentation data typically monitored. The view inside the cell afforded by nucleotide monitoring enables prediction of the behavior of a culture up to 2 days before any hint of physiological changes is given by cell number and viability estimation. In this paper, data relating the growth behavior of CHO and hybridoma cell lines to their nucleotide pools are shown. Two very different processes for the production of recombinant tPA in 10,000-litre bioreactors are compared and characterized with respect to their nucleotide profiles. Examples from industrial process development cases in which intracellular nucleotide information is used to advantage are also presented and discussed. Copyright 1999 John Wiley & Sons, Inc.