Flushing mechanism can be represented as a PK/PD interaction in which sildenafil exposure intersects with vascular signaling and cutaneous perfusion geometry. The mechanistic sequence begins with the concentration–time profile, then maps exposure into PDE5 modulation and downstream cGMP persistence within vascular smooth-muscle signaling. When this signaling alters vascular tone, the resulting vasodilation load can be represented as a change in regional perfusion geometry, including the cutaneous vascular compartment. The term flushing mechanism therefore refers here to modeled pathway activation rather than a clinical observation, incidence estimate, severity category, or subjective effect. Its geometry depends on the timing and magnitude of exposure, the relationship between sildenafil concentration and PDE5 modulation, and the responsiveness of the NO–cGMP signaling pathway. Early concentration formation determines when vascular modulation enters the rising phase, while subsequent exposure persistence determines how long the modeled pathway remains within its active concentration range. This framework treats cutaneous perfusion as a downstream expression of vascular signaling geometry. See vasodilation for the upstream vascular component.
The NO–cGMP pathway provides the principal signaling framework for describing sildenafil-associated vascular modulation. Nitric oxide activates soluble guanylyl cyclase, increasing intracellular cGMP formation within vascular smooth-muscle cells. PDE5 normally contributes to cGMP hydrolysis, so sildenafil-mediated PDE5 modulation changes the balance between cGMP formation and degradation. The resulting change in cGMP persistence alters the modeled signaling state downstream of nitric-oxide input. Flushing geometry can therefore be represented as a concentration-dependent interaction between sildenafil exposure and this pre-existing NO–cGMP system rather than as an independent process. The magnitude and timing of pathway modulation depend on sildenafil concentration, PDE5 occupancy relationships, cGMP turnover, and the temporal availability of upstream NO signaling. During the rising exposure phase, increasing sildenafil concentration progressively changes the modeled balance between PDE5 activity and cGMP persistence. During declining exposure, that modulation progressively relaxes as concentration falls. This produces a continuous PK→PD trajectory connecting exposure geometry with vascular signaling. The underlying signaling framework is described in no-cgmp and pde5 pathway.
Vasodilation load describes the modeled degree to which vascular signaling shifts toward reduced smooth-muscle contractile tone as sildenafil exposure modifies PDE5-dependent cGMP turnover. The relevant geometry is not a single event but a concentration-dependent pathway state. Increasing sildenafil exposure can increase PDE5 modulation, allowing cGMP signaling to persist differently from the baseline turnover pattern. Because vascular tone is coupled to cGMP signaling, this change can propagate into regional perfusion geometry, including the cutaneous vascular compartment. The flushing mechanism can therefore be modeled as an upstream signaling load that becomes progressively expressed as exposure enters and moves through the relevant concentration range. The temporal profile depends on the shape of the sildenafil concentration curve, while the downstream geometry depends on concentration–effect coupling within the vascular pathway. A rapidly rising concentration profile produces a steeper modeled transition into vascular modulation, whereas a slower exposure rise produces a more gradual transition. The same framework separates pathway activation from subjective interpretation by treating flushing only as a vascular signaling geometry. The broader pathway is detailed under vasodilation.
Distribution contributes to modeled flushing geometry by determining how sildenafil exposure moves between the central plasma compartment and peripheral tissues. After systemic entry, sildenafil is not represented as remaining exclusively within circulating plasma; instead, concentration gradients drive movement across interconnected compartments. The resulting distribution profile influences the temporal relationship between plasma exposure and tissue-level concentration available to interact with PDE5. For a cutaneous vascular compartment, the relevant construct is therefore local exposure geometry rather than a separately defined flushing process. The rate and extent of compartmental equilibration can shift the timing at which tissue exposure tracks the systemic concentration curve. Early distribution can modify the relationship between rising plasma concentrations and downstream vascular pathway activation, while subsequent equilibration contributes to the persistence and decline of tissue exposure. This produces a spatial component within the PK→PD model: systemic concentration provides the input, distribution determines compartmental availability, and local PDE5 modulation translates that availability into vascular signaling. The distribution framework therefore helps connect circulating exposure with modeled cutaneous perfusion geometry without treating the pathway as a clinical endpoint. See distribution for the underlying compartmental process.
Metabolism influences flushing-related pathway geometry by controlling how sildenafil exposure is transformed and subsequently removed from the systemic circulation. CYP3A4 is a major metabolic pathway for sildenafil, so variation in metabolic turnover can alter the rate at which circulating concentrations decline after the exposure peak. A faster metabolic component produces a steeper modeled reduction in available parent compound, whereas slower turnover allows the concentration trajectory to persist differently over time. The resulting exposure profile determines how long PDE5 modulation remains represented within the vascular PK→PD model. This creates a temporal connection between metabolic activity and cutaneous vasodilation geometry without requiring metabolism itself to be treated as a separate vascular mechanism. Variability in CYP3A4 activity can change the slope, persistence, and overall shape of the concentration–time curve, which then propagates through concentration–effect coupling. Clearance further determines the transition from sustained exposure toward declining exposure and pathway relaxation. The mechanistic emphasis is therefore on exposure persistence and decline rather than clinical duration or outcome interpretation. The relevant metabolic processes are described through metabolism and cyp3a4.
Absorption determines the initial input geometry from which sildenafil exposure develops, making it an important determinant of early modeled vascular pathway activation. Tablet dissolution first controls the availability of dissolved drug for subsequent gastrointestinal absorption, while gastric emptying influences the timing with which dissolved material reaches the principal absorptive region of the intestine. Absorption rate then determines how quickly sildenafil enters systemic circulation and begins forming the ascending concentration–time profile. A faster input process can produce a steeper early exposure curve, while a slower input process can broaden the rising phase. This difference propagates into PK→PD coupling because vascular PDE5 modulation depends on the concentration available at each point in time. The modeled flushing trajectory therefore begins upstream of the vascular compartment, with dissolution and gastrointestinal handling establishing the timing and shape of systemic input. Distribution subsequently determines compartmental availability, while concentration–effect coupling translates exposure into pathway modulation. The emphasis remains on the geometry of early exposure formation rather than clinical onset or subjective timing. The foundational absorption process is covered in absorption.
PK→PD coupling connects sildenafil exposure geometry to modeled vascular and cutaneous pathway activation. The PK component describes how sildenafil concentration changes over time through input, distribution, metabolism, and clearance, while the PD component describes how those concentrations interact with PDE5 and alter cGMP signaling. The resulting relationship is continuous rather than binary: changes in concentration produce corresponding changes in the modeled degree of PDE5 modulation, which then propagate through NO–cGMP signaling and vascular smooth-muscle tone. Cutaneous flushing geometry can consequently be represented as a downstream region of this exposure–response trajectory. During rising exposure, increasing concentration shifts the vascular signaling state progressively; near the peak, the pathway reflects the balance between ongoing input, distribution, and elimination; during declining exposure, decreasing concentration progressively reduces the modeled degree of PDE5 modulation. This framework separates pharmacokinetic timing from pharmacodynamic signaling while showing how the two domains interact. The final geometry is therefore generated by the coupling between concentration-time behavior and vascular concentration-effect relationships. The broader pharmacodynamic framework is summarized in pd summary.
PK variability produces a spread of modeled flushing trajectories because differences in absorption, distribution, metabolism, and clearance alter the underlying sildenafil concentration–time profile. Variability in absorption can change the slope and timing of the rising phase, while differences in distribution can alter the relationship between plasma concentration and peripheral tissue availability. Metabolic and clearance variability can then modify the declining phase, changing the persistence and relaxation of exposure-dependent PDE5 modulation. These PK differences propagate into PD behavior because the vascular pathway responds to the concentration trajectory rather than to a fixed time point. Consequently, two modeled exposure profiles can have different rising slopes, peak geometry, distribution relationships, and decline characteristics while passing through the same general NO–cGMP signaling framework. PD variability adds another layer by changing the mapping between concentration and pathway response, so identical PK profiles can theoretically generate different modeled signaling trajectories when concentration–effect coupling differs. The combined framework treats flushing geometry as the propagated output of interacting PK and PD variables rather than as a clinical endpoint. Relevant sources of PK spread are described through pk variability.
NO–cGMP amplification provides the upstream signaling context for modeled vasodilation load. Nitric oxide activates soluble guanylyl cyclase, increasing cGMP within vascular smooth-muscle cells, while PDE5 contributes to cGMP hydrolysis. Sildenafil modifies PDE5 activity, shifting the balance between cGMP formation and degradation. This changes the persistence of the intracellular signaling state generated by the NO–cGMP pathway. Within a flushing model, the resulting vascular geometry can be represented as an exposure-dependent change in smooth-muscle signaling and regional perfusion. The important variable is therefore the relationship between sildenafil concentration and PDE5 modulation, not a discrete clinical event. As concentration rises, the modeled degree of PDE5 modulation can increase; as concentration falls, that modulation progressively relaxes. The vascular response consequently follows the concentration–effect relationship and the kinetics of cGMP turnover. Cutaneous perfusion geometry emerges downstream when this vascular signaling state is expressed within peripheral vascular compartments. This mechanism links systemic exposure to regional perfusion without introducing clinical incidence or subjective interpretation. The underlying signaling architecture is described in no-cgmp.
Vasodilation load interacts with exposure timing because vascular pathway modulation follows the sildenafil concentration–time profile. During the early rising phase, increasing systemic concentration establishes progressively greater interaction with PDE5, shifting cGMP turnover and the associated vascular signaling state. The steepness of this transition depends on the shape of the exposure curve, which is influenced by the rate of systemic input and subsequent distribution. Near the upper portion of the concentration trajectory, the modeled vascular state reflects the balance between ongoing exposure formation and the concentration–effect relationship. During the declining phase, falling sildenafil concentration progressively reduces PDE5 modulation, allowing the signaling trajectory to move back toward its baseline turnover geometry. Cutaneous perfusion is therefore represented as a downstream spatial expression of a time-varying vascular pathway rather than as an independent mechanism. The same concentration profile can be analyzed as a sequence of input, exposure, PDE5 modulation, cGMP persistence, and vascular signaling states. This approach keeps the flushing construct within PK/PD geometry while avoiding clinical outcome interpretation. The broader vascular framework is covered under vasodilation.
| Domain | Mechanistic Determinant | Link |
|---|---|---|
| NO–cGMP Amplification | Upstream signaling. | no-cgmp |
| PDE5 Modulation | Vascular coupling. | vasodilation |
Distribution geometry determines how sildenafil exposure moves between the circulating compartment and peripheral tissue compartments. Following systemic entry, concentration gradients drive movement into tissues until compartmental equilibration is approached according to the relevant distribution kinetics. For a flushing-related model, this means that plasma concentration alone does not completely define the temporal availability of sildenafil within peripheral vascular regions. Tissue loading can lag behind, track, or redistribute relative to the central concentration curve depending on compartmental exchange rates and distribution characteristics. These relationships influence when peripheral PDE5 modulation is represented in the model and how closely local exposure follows the systemic profile. The resulting cutaneous exposure geometry is therefore a spatially resolved extension of the central PK trajectory. Early distribution contributes to the transition from circulating exposure toward peripheral availability, while later redistribution influences how concentrations evolve as systemic levels decline. This does not require a separate flushing pathway; instead, distribution modifies the exposure presented to the existing vascular signaling system. The mechanism is consequently represented as compartmental PK feeding into concentration-dependent PD modulation. The principal distribution process is described through distribution.
Redistribution provides a second component of cutaneous exposure geometry by describing how sildenafil concentrations continue to exchange among interconnected compartments after the initial distribution phase. As systemic concentrations change, concentration gradients can reverse or diminish, allowing drug movement between central and peripheral spaces to contribute to the shape of tissue exposure. In a modeled flushing pathway, this creates a temporal relationship between plasma decline and peripheral concentration decline rather than requiring both profiles to be identical. Peripheral exposure may therefore display a different slope or timing from the central compartment, depending on distribution kinetics. That difference can influence the duration and shape of local PDE5 modulation within the vascular model. Redistribution is especially relevant to the transition from peak exposure toward the declining phase because tissue release can contribute to continued peripheral availability while systemic concentrations fall. The resulting perfusion geometry remains downstream of concentration-dependent vascular signaling, but its temporal profile is partly shaped by compartmental exchange. This separates distribution effects from metabolic effects: redistribution changes where exposure resides, whereas metabolism changes the chemical persistence of sildenafil. The distribution framework is represented by distribution.
| Domain | Mechanistic Determinant | Link |
|---|---|---|
| Distribution Influence | Cutaneous exposure. | distribution |
| Redistribution | Perfusion persistence. | distribution |
CYP3A4 turnover shapes the metabolic component of sildenafil exposure persistence and therefore influences the time profile available for vascular PDE5 modulation. Metabolic conversion reduces the concentration of unchanged sildenafil available within the systemic compartment, contributing to the transition from peak exposure toward declining exposure. When CYP3A4 turnover is represented as faster, the parent-drug concentration trajectory can decline more rapidly; when turnover is slower, the decline can be represented as more gradual. These changes alter the duration and slope of the concentration-dependent vascular signaling trajectory without requiring metabolism itself to directly modify vascular tone. The flushing model therefore treats CYP3A4 as an upstream determinant of exposure persistence. Its influence enters through the concentration–time profile and propagates through PDE5 modulation, cGMP signaling, and vascular perfusion geometry. Variability in CYP3A4 activity can consequently produce different modeled exposure trajectories even when the initial input is similar. The relevant distinction is between metabolic transformation and downstream pathway activation: CYP3A4 determines how much parent compound remains available, while the vascular PD system determines how that concentration is translated into signaling. This metabolic component is detailed in cyp3a4.
Clearance geometry describes the overall decline of sildenafil exposure after systemic input and distribution, integrating metabolic removal with the resulting concentration trajectory. As clearance proceeds, circulating sildenafil concentration decreases, reducing the amount of parent compound available for PDE5 modulation. The vascular signaling state therefore follows the declining concentration profile rather than remaining fixed after the peak. A steeper clearance phase produces a more compressed modeled exposure decline, while a slower clearance phase broadens the declining trajectory. These differences can alter the persistence of the concentration-dependent NO–cGMP modulation represented within the model. Clearance should therefore be distinguished from distribution: distribution changes the spatial allocation of sildenafil, whereas clearance removes parent drug from the relevant systemic exposure pool. The resulting flushing geometry is determined by how these processes interact with the concentration–effect relationship governing PDE5 modulation. Metabolism contributes to clearance through chemical transformation, while other pharmacokinetic processes determine how the transformed exposure profile is distributed over time. This framework keeps the analysis focused on exposure decline, signaling persistence, and pathway relaxation rather than clinical duration or outcome. The broader clearance framework is represented through metabolism.
| Domain | Mechanistic Determinant | Link |
|---|---|---|
| CYP3A4 Turnover | Persistence constraints. | cyp3a4 |
| Clearance Geometry | Exposure decline. | metabolism |
Absorption rate establishes the early systemic input that precedes modeled vascular pathway activation. Once sildenafil becomes available for intestinal uptake, the rate at which drug enters the systemic circulation determines the slope of the initial concentration–time trajectory. A relatively rapid input process produces a steeper rising profile, whereas slower input spreads systemic entry across a broader interval. Because PDE5 modulation depends on sildenafil concentration, these differences in early exposure formation propagate into the timing and geometry of vascular signaling. The resulting flushing model therefore begins with the shape of the systemic input curve rather than with a separate cutaneous mechanism. Early concentration formation determines when sildenafil becomes available to interact with PDE5, while subsequent distribution determines its compartmental allocation. The vascular PD relationship then translates concentration into changes in cGMP turnover and modeled perfusion geometry. This sequence can be represented mathematically as an input function feeding a concentration–time model that is subsequently transformed through a concentration–effect relationship. The focus remains on the rising-phase geometry of exposure and its downstream signaling consequences. The underlying absorption process is described in absorption.
Dissolution and absorption form the upstream timing structure for early sildenafil exposure. Before systemic absorption can occur, the dosage form must release sildenafil into a dissolved state suitable for gastrointestinal uptake. Dissolution kinetics therefore influence the timing of available drug, while gastric emptying and intestinal transit affect when that available fraction reaches the primary absorptive region. Once absorption begins, the resulting input rate determines how quickly systemic concentration forms. The vascular pathway responds downstream to that concentration trajectory, so changes in dissolution or gastrointestinal timing can shift the modeled entry point into PDE5 modulation. A more concentrated early input can produce a steeper rising exposure curve, whereas dispersed input can broaden the ascending phase. This changes the temporal geometry of the subsequent concentration–effect relationship without requiring a separate flushing-specific mechanism. The pathway can therefore be represented as dosage-form release followed by gastrointestinal availability, systemic input, vascular distribution, PDE5 modulation, and downstream cGMP signaling. Each stage contributes a distinct timing function rather than duplicating the same PK process. The upstream absorption architecture is described through absorption deep dive.
| Domain | Mechanistic Determinant | Link |
|---|---|---|
| Absorption Rate | Early exposure. | absorption |
| Dissolution → Input | Upstream timing. | absorption deep dive |
Absorption variability changes the early input function and therefore modifies the rising geometry of modeled sildenafil exposure. Differences in dissolution, gastrointestinal transit, gastric emptying, or intestinal uptake can shift the timing and slope of systemic drug entry. In a PK model, these changes alter the ascending concentration–time curve before vascular PDE5 modulation becomes the dominant downstream relationship. A faster modeled input produces a steeper concentration rise, while slower or more dispersed input broadens the ascending phase. Because vascular signaling is concentration-dependent, the altered exposure curve subsequently changes the timing at which PDE5 modulation develops and the shape of the associated cGMP signaling trajectory. Absorption variability therefore propagates forward rather than acting directly on the cutaneous vascular compartment. The resulting modeled flushing trajectories can differ in their early slope, peak approach, and timing of downstream pathway activation even when the same mechanistic NO–cGMP architecture is used. This distinction keeps absorption variability separate from PD variability: the former changes drug input and concentration formation, while the latter changes how concentration is translated into signaling. The broader PK variability framework is described through pk variability.
Distribution and metabolism variability influence different portions of the sildenafil concentration profile and can therefore generate distinct forms of flushing-geometry variation. Distribution variability changes the movement of sildenafil between central and peripheral compartments, altering the relationship between plasma exposure and tissue availability. This can shift the timing or slope of peripheral exposure relative to the central concentration curve. Metabolism variability, by contrast, changes the rate at which parent sildenafil is transformed, modifying the declining phase and persistence of systemic exposure. Together, these processes determine how much sildenafil is represented within peripheral vascular compartments at different stages of the PK trajectory. Their effects propagate through PDE5 concentration–effect coupling into modeled cGMP signaling and vascular tone. A distribution difference can therefore primarily change spatial or compartmental timing, while a metabolic difference can primarily change exposure persistence and decline. Both ultimately alter the temporal geometry presented to the vascular PD system. This separation prevents distribution and metabolism from being treated as interchangeable determinants. The resulting variability is represented as a spread of mechanistic exposure trajectories rather than as a clinical variability measure. The combined PK framework is covered by pk variability.
PD variability adds a second layer to flushing-geometry variation by altering the mapping between sildenafil concentration and downstream vascular signaling. Two modeled profiles can share the same concentration–time curve while producing different degrees of PDE5 modulation if the concentration–effect relationship differs. This changes how exposure is translated into cGMP persistence, vascular smooth-muscle signaling, and regional perfusion geometry. PD variability can therefore affect the vertical response dimension of the model, while PK variability primarily changes the horizontal exposure trajectory through time. The two sources can also interact: an altered absorption or clearance profile changes the concentration input, while an altered concentration–effect relationship changes the downstream translation of that input. The resulting model is a coupled system in which variability can emerge at multiple stages, including exposure formation, tissue availability, metabolic decline, PDE5 interaction, and vascular signaling. This framework explains spread in modeled flushing trajectories without assigning clinical probability, severity, or outcome meaning. It instead describes how parameter variation changes the mathematical relationship between sildenafil exposure and vascular pathway activation. The relevant pharmacodynamic variability framework is described through pd variability.
| Variability Domain | Mechanistic Determinant | Link |
|---|---|---|
| Absorption Variability | Input variability. | pk variability |
| Distribution & Metabolism Variability | Exposure variability. | pk variability |
| PK → PD Variability | Propagation. | pd variability |
Mechanistically, sildenafil flushing can be represented as a downstream vascular pathway geometry produced by exposure-dependent PDE5 modulation and altered NO–cGMP signaling. Nitric oxide activates soluble guanylyl cyclase, generating cGMP, while PDE5 normally contributes to cGMP breakdown. Sildenafil modifies PDE5 activity, changing the persistence of cGMP signaling within vascular smooth-muscle pathways. The resulting change in vascular signaling can be represented as altered vascular tone and regional perfusion geometry, including a modeled cutaneous compartment. The PK component determines when and where sildenafil concentration is available through absorption, distribution, metabolism, and clearance. The PD component maps that concentration into PDE5 modulation and cGMP persistence. Flushing geometry therefore emerges from the coupling of concentration–time behavior with vascular concentration–effect relationships. It is not represented here as a subjective sensation, clinical endpoint, or outcome measure, but as a mechanistic expression of exposure-dependent vascular signaling.
Vasodilation contributes to modeled flushing geometry by providing the vascular pathway through which sildenafil-dependent PDE5 modulation can alter regional perfusion. Nitric oxide activates soluble guanylyl cyclase, increasing cGMP within vascular smooth-muscle cells. PDE5 normally contributes to cGMP hydrolysis, while sildenafil changes this turnover by inhibiting PDE5 activity. The resulting increase in cGMP persistence shifts the modeled signaling state toward reduced smooth-muscle contractile tone. When represented within cutaneous vascular compartments, this signaling change produces a corresponding perfusion geometry. The magnitude and timing of that geometry depend on sildenafil concentration, the concentration–effect relationship, cGMP turnover, and the temporal availability of upstream NO signaling. A rising concentration profile produces a progressive change in modeled pathway activation, while declining concentration produces progressive relaxation of the pathway state. Vasodilation is therefore a downstream PD expression of exposure-dependent PDE5 modulation rather than an independent flushing process. The complete geometry emerges from the interaction between PK exposure and vascular signaling.
Distribution influences cutaneous exposure by determining how sildenafil moves from the central circulating compartment into peripheral tissue compartments. After systemic entry, concentration gradients drive movement across interconnected spaces, producing a tissue exposure profile that does not necessarily mirror plasma concentration at every moment. The rate of distribution affects how quickly peripheral compartments approach their evolving concentration state, while redistribution can influence how those compartments change as systemic concentrations decline. In a flushing model, these compartmental relationships determine when sildenafil becomes available to interact with PDE5 within peripheral vascular regions. The resulting cutaneous exposure profile then feeds into concentration-dependent vascular signaling and cGMP modulation. Distribution therefore provides a spatial and temporal layer between systemic concentration and local pathway activation. It can alter the timing, slope, and persistence of modeled peripheral exposure without constituting a separate vascular mechanism. The relevant distinction is that distribution changes where sildenafil is available, whereas metabolism changes how rapidly parent sildenafil is transformed. Both processes modify the concentration presented to the vascular PD system.
Metabolism variability affects flushing geometry by changing the rate at which sildenafil is transformed and therefore altering the concentration–time profile available for PDE5 modulation. CYP3A4 contributes substantially to sildenafil metabolism, so differences in metabolic turnover can modify the declining portion of the exposure curve. Faster modeled turnover produces a more rapid reduction in parent-drug concentration, while slower turnover produces a more persistent concentration profile. Because PDE5 modulation is concentration-dependent, these metabolic differences propagate into the duration and slope of modeled vascular signaling. The resulting NO–cGMP trajectory can therefore differ even when the initial absorption profile is similar. Metabolism variability should be distinguished from distribution variability: distribution changes compartmental allocation, while metabolism changes chemical persistence of the parent compound. Clearance then integrates these processes into the overall exposure decline. The final flushing geometry is generated when this variable exposure profile is passed through the vascular concentration–effect relationship. Thus, metabolic variability changes the PK input to the vascular PD model rather than directly creating a separate cutaneous signaling pathway.
PK→PD coupling explains modeled flushing geometry by connecting sildenafil concentration over time with downstream PDE5 and vascular signaling. Pharmacokinetics determines the exposure trajectory through processes such as absorption, distribution, metabolism, and clearance. Pharmacodynamics then maps that concentration into PDE5 modulation and the resulting change in cGMP turnover. As sildenafil concentration rises, the modeled degree of PDE5 modulation changes progressively; as concentration declines, the modulation relaxes according to the concentration–effect relationship. The vascular signaling state can then be represented within a cutaneous compartment as a corresponding perfusion geometry. This creates a continuous pathway from systemic exposure to local vascular signaling without treating flushing as an independent endpoint. Differences in absorption rate can alter the rising phase, distribution can alter peripheral availability, and metabolism can alter the declining phase. PD variability can additionally modify how a given concentration translates into signaling. The resulting geometry is therefore the product of coupled PK and PD functions, with exposure serving as the time-varying input and vascular signaling providing the downstream mechanistic response.