Is it possible that invitro inhibition of alpha glucosidase enzyme always show antidiabetic effect invivo. Does invitro inhibition of alpha glucosidase enzyme always give positive antidiabetic result invivo.
In order for a drug of any kind to work in vivo, it is necessary for it to reach a certain concentration for a certain amount of time. These pharmacokinetic properties are separate parameters from inhibition of the target, so there is no guarantee that the in vitro alpha-glucosidase inhibitor will achieve an in vivo effect. For example, it may have too low oral bioavailability, or it may be cleared from the blood too quickly. The properties that affect the blood level of a drug are referred to as ADME (absorption, distribution, metabolism, elimination).
Alpha glucosidase inhibitor exerts their effect topically i.e. intestine and the systemic availability of the drug therefore has no effect on desired therapeutic effect.
Characterizing alpha-glucosidase inhibitor as having "antidiabetic effect" is a bit misleading. As you correctly state, its effect is primarily in the intestine - limiting cleavage of glucose from polysaccharides. It is more like creating selective malabsorption. Eating less or a diet low on carbohydrates will accomplish the same thing with no side effects of the inhibitor... and there are a fair few.
Alpha glucosidase inhibition does not equate to antidiabetic effect, although in the case of herbs this may be a pointer that said herb could be be useful in preventing post pandrial hyperglcaemia which is an important factor in glucose control.
In-vitro alpha glucosidase inhibition may not necessarily equate antidiabetic activity in-vivo. In fact, in-vivo, alpha glucosidase inhibition is not one of the major mechanisms for achieving blood glucose control in diabetics, it is regarded as an adjunct.
If inhibition of alpha-glucosidase occurs in the digestive tract, then inhibition by the drug depends on its concentration there. If the drug is broken down in the digestive tract into an inactive product, or if it is taken up from the digestive tract by the tissues, then the concentration at the site of activation will decrease, reducing the effectiveness of the drug.