  The notion of lasing with biologics has recently been realized and has since
rapidly developed with the collective objective of creating lasers $\textit{in
vivo}$. One limitation of achieving this goal is the requirement of exogenous
laser dyes and fluorescent materials. To circumvent this, we investigate the
use of cell-endogenous fluorophores - sources of cell autofluorescence - as
laser gain material. In this work, we study the lasing potential and efficiency
of flavins and reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H)
using a dye lasing model based on coupled rate equations. Analytical solutions
for one- and two-photon pumped system were used in multi-parameter studies. We
found that at physiological conditions, lasing can be supported by NAD(P)H with
cavity quality factors of $10^5$. With the further consideration of damage
thresholds, we recommend the use of flavins as they entail lower threshold
requirements. We then identify potential parameters for engineering to make the
lasing of flavins feasible even at their low physiological intracellular
concentrations. We also note the higher threshold requirements and lower
efficiencies of two-photon pumping, but recognize its potential for realizing
lasing $\textit{in vivo}$.
