Here, we explore the in vivo effects of four weeks AbMole Folic acid treatment with EtPhCbl using a mouse model. We report that EtPhCbl is able to induce a Cbl deficiency and at the same time impair the cellular internalization of Cbl. Recently, we described a new organometallic form of Cbl, EtPhCbl, which was inert against the action of the crucial enzyme CblC in in vitro enzyme assays, and therefore would not undergo a later conversion to the biological active AdoCbl and CH 3Cbl. Here, we report that mice treated with EtPhCbl for four weeks show signs of an impaired metabolism of Cbl characterized by tissue depletion of Cbl and an increased level of the metabolic markers of Cbl deficiency, MMA and tHCy. The cellular AbMole 3,4,5-Trimethoxyphenylacetic acid uptake of Cbl is mediated by TC, and only forms of Cbl recognized by TC can enter the cells using this transport system. Under physiological conditions the TC transport system is far from saturated. This has two implications: Firstly treatment with CNCbl – as well as other forms of Cbl – results in tissue accumulation of the vitamin ; Secondly the TC transport system has sufficient capacity for transporting pharmacological Cbl derivatives into the cells without impairing the transport of endogenous Cbl. In accord with this, we recently reported tissue accumulation upon loading mice with cobinamide, an inactive form of Cbl recognized by mouse TC. Since EtPhCbl binds to TC with an affinity comparable to that of H2OCbl and CNCbl, we expected to see tissue levels of EtPhCbl comparable to those observed when treating the animals with the same amounts of CNCbl. Unexpectedly, this was not the case. After four weeks of treatment with EtPhCbl, the total Cbl levels were high in plasma and low in the tissues compared to the mice treated with the same dose of CNCbl. Currently, we do not know whether the low level of tissue Cbl represents an impaired uptake or an increased export of EtPhCbl. Regardless of the mechanism, however, our observation has implications for the pharmacological use of the TC transport system. Several attempts have been made in order to use the TC mediated cellular uptake for carrying e.g. drugs into the cells. Our results emphasize that the success of such an attempt depends not only on the binding of the Cbl derivative to TC, but also on the transport of the Cbl derivative into the cells and its ability to remain in the cells. Furthermore, this must occur without impairing the accumulation of endogenous Cbl. Our mouse model may prove very useful in exploring whether TC bound Cbl derivatives are accumulated in the tissues without impairing the accumulation of endogenous Cbl. Such studies are of obvious importance prior to introducing a new Cbl derivative in the clinical setting. Interestingly, Cbl depletion upon treatment with EtPhCbl showed distinct differences between the various tissues studied. The most severe Cbl depletion was observed in the liver and the spleen, while the spinal cord was affected only to a limited degree. In addition, accumulation of EtPhCbl varied between the tissues. While almost all of the Cbl present in the kidney was EtPhCbl, nothing was observed in the spinal cord. Our results suggest tissue-specific differences related to the handling of Cbl. Such differences have previously been described for the kidney as compared to other tissues. The kidney has a dual Cbl uptake system. TC is recognized not only by its receptor, CD320, but, after filtration, also by the multifunctional receptor, megalin, present on the luminal membranes of the proximal tubules. To our knowledge, little is known about differences in Cbl handling of the other organs studied.