Final answer:
The fifth barrier a drug must pass to reach its target organ, particularly in the central nervous system, is the blood-brain barrier (BBB). It is crucial for drugs targeting the CNS to be able to cross this barrier to have the desired effect.
Step-by-step explanation:
The fifth barrier that a drug must pass to get to its target organ is the blood-brain barrier (BBB). This physiological barrier keeps many substances that circulate in the rest of the body from getting into the central nervous system (CNS), thus affecting drug delivery to the CNS. Nutrient molecules like glucose or amino acids can pass through the BBB, but other molecules cannot. Pharmaceutical companies face the challenge of designing drugs that not only cross the BBB but also effectively target CNS functions.
To successfully enter the CNS and exert the desired therapeutic effect, a drug must navigate through several barriers. These include the cellular membrane barrier, metabolic enzymes, efflux transporters like P-glycoprotein (PgP), and plasma protein binding (PPB). The blood-brain barrier represents a significant hurdle due to its restrictive properties, which prevent many compounds, except for certain nutrients, from penetrating. Strategies to overcome this include modifying the drug's structure to reduce PgP efflux or adjusting its physiochemical properties to enhance penetration.