12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

STRIBILD is a fixed-dose combination of antiretroviral drugs elvitegravir (boosted by the CYP3A inhibitor cobicistat), emtricitabine, and TDF [see Microbiology (12.4)].

12.2 Pharmacodynamics

Effects on Electrocardiogram

Thorough QT studies have been conducted for elvitegravir and cobicistat. The effect of the other two components, tenofovir and emtricitabine, or the combination regimen STRIBILD on the QT interval is not known.

Elvitegravir: In a thorough QT/QTc study in 126 healthy subjects, elvitegravir (coadministered with 100 mg ritonavir) 125 mg and 250 mg (0.83 and 1.67 times the dose in STRIBILD) did not affect the QT/QTc interval and did not prolong the PR interval.

Cobicistat: In a thorough QT/QTc study in 48 healthy subjects, a single dose of cobicistat 250 mg and 400 mg (1.67 and 2.67 times the dose in STRIBILD) did not affect the QT/QTc interval. Prolongation of the PR interval was noted in subjects receiving cobicistat. The maximum mean (95% upper confidence bound) difference in PR from placebo after baseline-correction was 9.5 (12.1) msec for the 250 mg cobicistat dose and 20.2 (22.8) for the 400 mg cobicistat dose. Because the 150 mg cobicistat dose used in the STRIBILD fixed-dose combination tablet is lower than the lowest dose studied in the thorough QT study, it is unlikely that treatment with STRIBILD will result in clinically relevant PR prolongation.

Effects on Serum Creatinine

The effect of cobicistat on serum creatinine was investigated in a Phase 1 study in subjects with an eGFR of at least 80 mL per minute (N=18) and with an eGFR of 50 to 79 mL per minute (N=12). A statistically significant change of eGFRCG from baseline was observed after 7 days of treatment with cobicistat 150 mg among subjects with an eGFR of at least 80 mL per minute (−9.9 ± 13.1 mL/min) and subjects with an eGFR of 50 to 79 mL per minute (−11.9 ± 7.0 mL per minute). These decreases in eGFRCG were reversible after cobicistat was discontinued. The actual glomerular filtration rate, as determined by the clearance of probe drug iohexol, was not altered from baseline following treatment of cobicistat among subjects with an eGFR of at least 50 mL per minute, indicating cobicistat inhibits tubular secretion of creatinine, reflected as a reduction in eGFRCG, without affecting the actual glomerular filtration rate.

12.3 Pharmacokinetics

The pharmacokinetic properties of the components of STRIBILD are provided in Table 6. The multiple dose pharmacokinetic parameters of elvitegravir, cobicistat, emtricitabine, and tenofovir are provided in Table 7.

Table 6 Pharmacokinetic Properties of the Components of STRIBILD
	Elvitegravir	Cobicistat	Emtricitabine	Tenofovir
NC=Not Calculated
* Values refer to mean systemic exposure (90% confidence interval). STRIBILD light meal=~373 kcal, 20% fat; STRIBILD high fat meal=~800 kcal, 50% fat. Increase = ↑; Decrease = ↓ † t1/2 values refer to median terminal plasma half-life. ‡ Dosing in mass balance studies: elvitegravir (single dose administration of [14C] elvitegravir, coadministered with 100 mg RTV); cobicistat (single-dose administration of [14C] cobicistat after multiple dosing of cobicistat for six days); emtricitabine (single dose administration of [14C] emtricitabine after multiple dosing of emtricitabine for ten days); mass balance study not conducted for tenofovir.
Absorption
Tmax (h)	4	3	3	2
Effect of light meal (relative to fasting)*	↑34% (↑19, ↑51)	↑3% (↓10, ↑17)	↓5% (↓9, 0)	↑24% (↑18, ↑30)
Effect of high fat meal (relative to fasting)*	↑87% (↑66, ↑110)	↓17% (↓27, ↓5)	↓4% (↓8, 0)	↑23% (↑17, ↑29)
Distribution
% Bound to human plasma proteins	~99	~98	<4	<0.7
Source of protein binding data	Ex vivo	In vitro	In vitro	In vitro
Blood-to-plasma ratio	0.73	0.5	0.6	NC
Metabolism
Metabolism	CYP3A (major) UGT1A1/3 (minor)	CYP3A (major) CYP2D6 (minor)	Not significantly metabolized
Elimination
Major route of elimination	Metabolism		Glomerular filtration and active tubular secretion
T1/2 (h)†	12.9	3.5	10	12–18
% Of dose excreted in urine‡	6.7	8.2	70	70–80
% Of dose excreted in feces‡	94.8	86.2	13.7	NC

Table 7 Pharmacokinetic Parameters of Elvitegravir, Cobicistat, Emtricitabine, and Tenofovir Exposure Following Oral Administration of STRIBILD in HIV-Infected Subjects
Parameter Mean ± SD [range, min:max]	Elvitegravir*	Cobicistat†	Emtricitabine†	Tenofovir†
SD=Standard Deviation
* From Population Pharmacokinetic analysis, N=419. † From Intensive Pharmacokinetic analysis, N=61–62, except cobicistat Ctrough N=53.
Cmax (microgram per mL)	1.7 ± 0.4 [0.4:3.7]	1.1 ± 0.4 [0.1:2.1]	1.9 ± 0.5 [0.6:3.6]	0.45 ± 0.2 [0.2:1.2]
AUCtau (microgram∙hour per mL)	23.0 ± 7.5 [4.4:69.8]	8.3 ± 3.8 [0.5:18.3]	12.7 ± 4.5 [5.2:34.1]	4.4 ± 2.2 [2.1:18.2]
Ctrough (microgram per mL)	0.45 ± 0.26 [0.05:2.34]	0.05 ± 0.13 [0.01:0.92]	0.14 ± 0.25 [0.04:1.94]	0.10 ± 0.08 [0.04:0.58]

Specific Populations

Geriatric Patients

The pharmacokinetics of elvitegravir, cobicistat, emtricitabine, and tenofovir have not been fully evaluated in elderly (65 years of age and older) patients [see Use in Specific Populations (8.5)].

Pediatric Patients

Exposures (AUC) of elvitegravir and tenofovir in 14 pediatric subjects aged 12 to less than 18 years who received STRIBILD in Study 112 were increased by 30% and 37%, respectively, compared with exposures achieved in adults following administration of STRIBILD, but were deemed acceptable based on the overall safety profile of these agents and exposure-safety assessments. The other components of STRIBILD had similar exposures in adolescents compared with adults [see Use in Specific Populations (8.4)].

Emtricitabine has been studied in pediatric subjects from 3 months to 17 years of age. TDF has been studied in pediatric subjects from 2 years to less than 18 years of age. The pharmacokinetics of elvitegravir or cobicistat in pediatric subjects less than 12 years of age have not been established [see Use in Specific Populations (8.4)].

Race, Gender

No clinically significant differences in pharmacokinetics of STRIBILD have been identified based on race or gender.

Patients with Renal Impairment

Elvitegravir and Cobicistat: A study of the pharmacokinetics of cobicistat+elvitegravir was performed in healthy subjects and subjects with severe renal impairment (estimated creatinine clearance less than 30 mL per minute). No clinically relevant differences in elvitegravir or cobicistat pharmacokinetics were observed between healthy subjects and subjects with severe renal impairment.

Emtricitabine and TDF: The pharmacokinetics of emtricitabine and tenofovir are altered in subjects with estimated creatinine clearance below 50 mL per minute or with end-stage renal disease requiring dialysis (ESRD) (Table 8) [see Warnings and Precautions (5.2) and Use in Specific Populations (8.6)].

Table 8 Pharmacokinetic Parameters of Emtricitabine* and Tenofovir† in Adults with Varying Degrees of Renal Function
Parameter Mean ± SD	Creatinine Clearance (mL/min)
Parameter Mean ± SD	>80	50–80	30–49	<30	ESRD‡
SD=Standard Deviation
* 200 mg, single dose of emtricitabine † 300 mg, single dose of TDF ‡ ESRD subjects requiring dialysis
Emtricitabine	N=6	N=6	N=6	N=5	N=5
AUCinf (microgram∙hr per mL)	11.8 ± 2.9	19.9 ± 1.2	25.1 ± 5.7	33.7± 2.1	53.2 ± 9.9
Cmax (microgram per mL)	2.2 ± 0.6	3.8 ± 0.9	3.2 ± 0.6	2.8 ± 0.7	2.8 ± 0.5
Tenofovir	N=3	N=10	N=8	N=11	N=9
AUCinf (microgram∙hr per mL)	2.18 ± 0.26	3.06 ± 0.93	6.01 ± 2.50	15.98 ± 7.22	44.90 ± 12.96
Cmax (microgram per mL)	0.34 ± 0.03	0.33 ± 0.06	0.37 ± 0.16	0.60 ± 0.19	1.06 ± 0.25

Patients with Hepatic Impairment

Elvitegravir and Cobicistat: A study of the pharmacokinetics of cobicistat+elvitegravir was performed in healthy subjects and subjects with moderate hepatic impairment (Child-Pugh Class B). No clinically relevant differences in elvitegravir or cobicistat pharmacokinetics were observed between subjects with moderate hepatic impairment and healthy subjects. The effect of severe hepatic impairment (Child-Pugh Class C) on the pharmacokinetics of elvitegravir or cobicistat has not been studied [see Use in Specific Populations (8.7)].

Emtricitabine: The pharmacokinetics of emtricitabine have not been studied in subjects with hepatic impairment; however, emtricitabine is not significantly metabolized by liver enzymes, so the impact of liver impairment should be limited.

Tenofovir DF: The pharmacokinetics of tenofovir following a 300 mg dose of VIREAD have been studied in healthy subjects with moderate to severe hepatic impairment (Child-Pugh Class C). No clinically relevant differences in tenofovir pharmacokinetics were observed between subjects with hepatic impairment and healthy subjects.

Hepatitis B and/or Hepatitis C Virus Coinfection

Elvitegravir: Limited data from population pharmacokinetic analysis (N=24) indicated that hepatitis B and/or C virus infection had no clinically relevant effect on the exposure of cobicistat-boosted elvitegravir.

Cobicistat: There were insufficient pharmacokinetic data in the clinical trials to determine the effect of hepatitis B and/or C virus infection on the pharmacokinetics of cobicistat.

Emtricitabine and TDF: The pharmacokinetics of emtricitabine and TDF have not been fully evaluated in subjects coinfected with hepatitis B and/or C virus.

Assessment of Drug Interactions

[see Contraindications (4) and Drug Interactions (7)]

The drug-drug interaction studies described were conducted with STRIBILD, elvitegravir (coadministered with cobicistat or RTV), or cobicistat administered alone.

As STRIBILD is indicated for use as a complete regimen for the treatment of HIV-1 infection and should not be administered with other antiretroviral medications, information regarding drug-drug interactions with other antiretroviral agents is not provided.

The effects of coadministered drugs on the exposure of elvitegravir, emtricitabine, and tenofovir are shown in Table 9, Table 10, and Table 11 respectively. The effects of elvitegravir plus cobicistat, or cobicistat, or emtricitabine on the exposure of coadministered drugs are shown in Table 12. For information regarding clinical recommendations, [see Drug Interactions (7)].

Table 9 Drug Interactions: Changes in Pharmacokinetic Parameters for Elvitegravir in the Presence of the Coadministered Drug*
Coadministered Drug	Dose of Coadministered Drug	Elvitegravir Dose (mg)	Cobicistat or RTV Booster Dose (mg)	N	Mean Ratio of Elvitegravir Pharmacokinetic Parameters (90% CI); No Effect=1.00
Coadministered Drug	Dose of Coadministered Drug	Elvitegravir Dose (mg)	Cobicistat or RTV Booster Dose (mg)	N	Cmax	AUC	Cmin
* All interaction studies conducted in healthy volunteers. † Maximum strength antacid contained 80 mg aluminum hydroxide, 80 mg magnesium hydroxide, and 8 mg simethicone, per mL. ‡ Study conducted with GENVOYA® (elvitegravir/cobicistat/emtricitabine/tenofovir alafenamide). § Percent change of cobicistat PK parameters (90% CI) was 1.25 (1.18 to 1.32) for Cmax, 1.59 (1.49 to 1.70) for AUC, and 4.25 (3.47 to 5.22) for Cmin. ¶ Study conducted with STRIBILD. # Percent change of cobicistat PK parameters (90% CI) was1.11 (1.06, 1.17) for Cmax, 1.23 (1.17, 1.29) for AUC, and 1.71 (1.54, 1.90) for Cmin. Þ Study conducted with additional voxilaprevir 100 mg to achieve voxilaprevir exposures expected in HCV-infected patients.
Maximum strength antacid†	20 mL single dose given 4 hours before elvitegravir	50 single dose	RTV 100 single dose	8	0.95 (0.84, 1.07)	0.96 (0.88, 1.04)	1.04 (0.93, 1.17)
	20 mL single dose given 4 hours after elvitegravir			10	0.98 (0.88, 1.10)	0.98 (0.91, 1.06)	1.00 (0.90, 1.11)
	20 mL single dose given 2 hours before elvitegravir			11	0.82 (0.74, 0.91)	0.85 (0.79, 0.91)	0.90 (0.82, 0.99)
	20 mL single dose given 2 hours after elvitegravir			10	0.79 (0.71, 0.88)	0.80 (0.75, 0.86)	0.80 (0.73, 0.89)
Atorvastatin	10 mg single dose	150 once daily‡	Cobicistat 150 once daily‡	16	0.91 (0.85, 0.98)	0.92 (0.87, 0.98)	0.88 (0.81, 0.96)
Carbamazepine	200 mg twice daily	150 once daily	Cobicistat 150 once daily	12	0.55 (0.49, 0.61)	0.31 (0.28, 0.33)	0.03 (0.02, 0.04)
Famotidine	40 mg once daily given 12 hours after elvitegravir	150 once daily	Cobicistat 150 once daily	10	1.02 (0.89, 1.17)	1.03 (0.95, 1.13)	1.18 (1.05, 1.32)
Famotidine	40 mg once daily given simultaneously with elvitegravir	150 once daily	Cobicistat 150 once daily	16	1.00 (0.92, 1.10)	1.03 (0.98, 1.08)	1.07 (0.98, 1.17)
Ketoconazole	200 mg twice daily	150 once daily	RTV 100 once daily	18	1.17 (1.04, 1.33)	1.48 (1.36, 1.62)	1.67 (1.48, 1.88)
Ledipasvir/Sofosbuvir	90/400 mg once daily	150 once daily	Cobicistat 150 once daily§	29	0.88 (0.82, 0.95)	1.02 (0.95, 1.09)	1.36 (1.23, 1.49)
Omeprazole	40 mg once daily given 2 hours before elvitegravir	50 once daily	RTV 100 once daily	9	0.93 (0.83, 1.04)	0.99 (0.91, 1.07)	0.94 (0.85, 1.04)
	20 mg once daily given 2 hours before elvitegravir	150 once daily	Cobicistat 150 once daily	11	1.16 (1.04, 1.30)	1.10 (1.02, 1.19)	1.13 (0.96, 1.34)
	20 mg once daily given 12 hours after elvitegravir	150 once daily	Cobicistat 150 once daily	11	1.03 (0.92, 1.15)	1.05 (0.93, 1.18)	1.10 (0.92, 1.32)
Rifabutin	150 mg once every other day	150 once daily	Cobicistat 150 once daily	12	0.91 (0.84, 0.99)	0.79 (0.74, 0.85)	0.33 (0.27, 0.40)
Rosuvastatin	10 mg single dose	150 once daily	Cobicistat 150 once daily	10	0.94 (0.83, 1.07)	1.02 (0.91, 1.14)	0.98 (0.83, 1.16)
Sertraline	50 mg single dose	150 once daily‡	Cobicistat 150 once daily‡	19	0.88 (0.82, 0.93)	0.94 (0.89, 0.98)	0.99 (0.93, 1.05)
Sofosbuvir/Velpatasvir	400/100 mg once daily	150 once daily¶	Cobicistat 150 once daily¶,#	24	0.93 (0.86, 1.00)	0.93 (0.87, 0.99)	0.97 (0.91, 1.04)
Sofosbuvir/Velpatasvir Voxilaprevir	400/100/100 + 100 Voxilaprevir once dailyÞ	150 once daily‡	Cobicistat 150 once daily‡	29	0.79 (0.75, 0.85)	0.94 (0.88, 1.00)	1.32 (1.17, 1.49)

Table 10 Drug Interactions: Changes in Pharmacokinetic Parameters for Emtricitabine in the Presence of the Coadministered Drug*
Coadministered Drug	Dose of Coadministered Drug (mg)	Emtricitabine Dose (mg)	N	Mean Ratio of Emtricitabine Pharmacokinetic Parameters (90%CI); No Effect=1.00
Coadministered Drug	Dose of Coadministered Drug (mg)	Emtricitabine Dose (mg)	N	Cmax	AUC	Cmin
NC=Not calculated
* All interaction studies conducted in healthy volunteers.
Famciclovir	500 single dose	200 single dose	12	0.90 (0.80, 1.01)	0.93 (0.87, 0.99)	NC

Table 11 Drug Interactions: Changes in Pharmacokinetic Parameters for Tenofovir in the Presence of the Coadministered Drug*
Coadministered Drug	Dose of Coadministered Drug (mg)	TDF Dose (mg)	N	Mean Ratio of Tenofovir Pharmacokinetic Parameters (90%CI); No Effect=1.00
Coadministered Drug	Dose of Coadministered Drug (mg)	TDF Dose (mg)	N	Cmax	AUC	Cmin
* All interaction studies conducted in healthy volunteers. † Study conducted with STRIBILD.
Sofosbuvir/Velpatasvir	400/100 once daily	300 once daily†	24	1.36 (1.25, 1.47)	1.35 (1.29, 1.42)	1.45 (1.39, 1.51)

Table 12 Drug Interactions: Changes in Pharmacokinetic Parameters for Coadministered Drug in the Presence of Elvitegravir plus Cobicistat, Cobicistat, Emtricitabine, or STRIBILD*
Coadministered Drug	Dose of Coadministered Drug (mg)	Elvitegravir Dose† (mg)	Cobicistat Booster Dose (mg)	FTC Dose (mg)	N	Mean Ratio of Coadministered Drug Pharmacokinetic Parameters‡ (90% CI); No Effect=1.00
Coadministered Drug	Dose of Coadministered Drug (mg)	Elvitegravir Dose† (mg)	Cobicistat Booster Dose (mg)	FTC Dose (mg)	N	Cmax	AUC	Cmin
FTC=emtricitabine
* All interaction studies conducted in healthy volunteers. † NA=Not Applicable ‡ NC=Not Calculated § Study conducted with GENVOYA. ¶ The predominant circulating nucleoside metabolite of sofosbuvir. # Study conducted with STRIBILD. Þ Study conducted with additional voxilaprevir 100 mg to achieve voxilaprevir exposures expected in HCV-infected patients ß Comparison based on rifabutin 300 mg once daily.
Atorvastatin	10 single dose	150 once daily§	150 once daily§	200 once daily§	16	2.32 (1.91, 2.82)	2.60 (2.31, 2.93)	NC
Buprenorphine	16–24 once daily	150 once daily	150 once daily	NA	17	1.12 (0.98, 1.27)	1.35 (1.18, 1.55)	1.66 (1.43, 1.93)
Norbuprenorphine	16–24 once daily	150 once daily	150 once daily	NA	17	1.24 (1.03, 1.49)	1.42 (1.22, 1.67)	1.57 (1.31, 1.88)
Carbamazepine	200 twice daily	150 once daily	150 once daily	NA	12	1.40 (1.32, 1.49)	1.43 (1.36, 1.52)	1.51 (1.41, 1.62)
Carbamazepine-10,11-epoxide	200 twice daily	150 once daily	150 once daily	NA	12	0.73 (0.70, 0.78)	0.65 (0.63, 0.66)	0.59 (0.57, 0.61)
Desipramine	50 single dose	NA	150 once daily	NA	8	1.24 (1.08, 1.44)	1.65 (1.36, 2.02)	NC
Digoxin	0.5 single dose	NA	150 once daily	NA	22	1.41 (1.29, 1.55)	1.08 (1.00, 1.17)	NC
Famciclovir	500 single dose	NA	NA	200 single dose	12	0.93 (0.78, 1.11)	0.91 (0.84, 0.99)	NC
Ledipasvir	90/400 once daily	150 once daily	150 once daily	NA	29	1.63 (1.51, 1.75)	1.78 (1.64, 1.94)	1.91 (1.76, 2.08)
Sofosbuvir						1.33 (1.14, 1.56)	1.36 (1.21, 1.52)	NA
GS-331007¶						1.33 (1.22, 1.44)	1.44 (1.41, 1.48)	1.53 (1.47, 1.59)
Naloxone	4–6 once daily	150 once daily	150 once daily	NA	17	0.72 (0.61, 0.85)	0.72 (0.59, 0.87)	NA
Norgestimate/ ethinyl estradiol	0.180/0.215/ 0.250 norgestimate once daily	150 once daily#	150 once daily#	200 once daily#	13	2.08 (2.00, 2.17)	2.26 (2.15, 2.37)	2.67 (2.43, 2.92)
Norgestimate/ ethinyl estradiol	0.025 ethinyl estradiol once daily	150 once daily#	150 once daily#	200 once daily#	13	0.94 (0.86, 1.04)	0.75 (0.69, 0.81)	0.56 (0.52, 0.61)
R-Methadone	80–120 daily	150 once daily	150 once daily	NA	11	1.01 (0.91, 1.13)	1.07 (0.96, 1.19)	1.10 (0.95, 1.28)
S-Methadone	80–120 daily	150 once daily	150 once daily	NA	11	0.96 (0.87, 1.06)	1.00 (0.89, 1.12)	1.02 (0.89, 1.17)
Sofosbuvir	400/100 once daily	150 once daily#	150 once daily#	200 once daily#	24	1.01 (0.85, 1.19)	1.24 (1.13, 1.37)	NA
GS-331007¶						1.13 (1.07, 1.18)	1.35 (1.30, 1.40)	1.45 (1.38, 1.52)
Velpatasvir						1.05 (0.93, 1.19)	1.19 (1.07, 1.34)	1.37 (1.22, 1.54)
Sofosbuvir	400/100/100 + 100 VoxilaprevirÞ once daily	150 once daily§	150 once daily§	200 once daily§	29	1.27 (1.09, 1.48)	1.22 (1.12, 1.32)	NC
GS-331007¶						1.28 (1.25, 1.32)	1.43 (1.39, 1.47)	NC
Velpatasvir						0.96 (0.89, 1.04)	1.16 (1.06, 1.27)	1.46 (1.30, 1.64)
Voxilaprevir						1.92 (1.63, 2.26)	2.71 (2.30, 3.19)	4.50 (3.68, 5.50)
Rifabutin	150 once every other day	150 once daily	150 once daily	NA	12	1.09 (0.98, 1.20)ß	0.92 (0.83, 1.03)ß	0.94 (0.85, 1.04)ß
25-O-desacetyl-rifabutin	150 once every other day	150 once daily	150 once daily	NA	12	4.84 (4.09, 5.74)ß	6.25 (5.08, 7.69)ß	4.94 (4.04, 6.04)ß
Rosuvastatin	10 single dose	150 once daily	150 single dose	NA	10	1.89 (1.48, 2.42)	1.38 (1.14, 1.67)	NC
Sertraline	50 single dose	150 once daily§	150 once daily§	200 once daily§	19	1.14 (0.94, 1.38)	0.93 (0.77, 1.13)	NA

12.4 Microbiology

Mechanism of Action

Elvitegravir: Elvitegravir inhibits the strand transfer activity of HIV-1 integrase (integrase strand transfer inhibitor; INSTI), an HIV-1 encoded enzyme that is required for viral replication. Inhibition of integrase prevents the integration of HIV-1 DNA into host genomic DNA, blocking the formation of the HIV-1 provirus and propagation of the viral infection. Elvitegravir does not inhibit human topoisomerases I or II.

Cobicistat: Cobicistat is a selective, mechanism-based inhibitor of cytochromes P450 of the CYP3A subfamily. Inhibition of CYP3A-mediated metabolism by cobicistat enhances the systemic exposure of CYP3A substrates, such as elvitegravir, where bioavailability is limited and half-life is shortened by CYP3A-dependent metabolism.

Emtricitabine: Emtricitabine, a synthetic nucleoside analog of cytidine, is phosphorylated by cellular enzymes to form emtricitabine 5'-triphosphate. Emtricitabine 5'-triphosphate inhibits the activity of the HIV-1 RT by competing with the natural substrate deoxycytidine 5'-triphosphate and by being incorporated into nascent viral DNA, which results in chain termination. Emtricitabine 5'-triphosphate is a weak inhibitor of mammalian DNA polymerases α, β, ε, and mitochondrial DNA polymerase γ.

Tenofovir DF: Tenofovir DF is an acyclic nucleoside phosphonate diester analog of adenosine monophosphate. Tenofovir DF requires initial diester hydrolysis for conversion to tenofovir and subsequent phosphorylations by cellular enzymes to form tenofovir diphosphate. Tenofovir diphosphate inhibits the activity of HIV-1 RT by competing with the natural substrate deoxyadenosine 5'-triphosphate and, after incorporation into DNA, by DNA chain termination. Tenofovir diphosphate is a weak inhibitor of mammalian DNA polymerases α, β, and mitochondrial DNA polymerase γ.

Antiviral Activity in Cell Culture

Elvitegravir, Cobicistat, Emtricitabine, and TDF: The triple combination of elvitegravir, emtricitabine, and tenofovir was not antagonistic in cell culture combination antiviral activity assays and was not affected by the addition of cobicistat.

Elvitegravir: The antiviral activity of elvitegravir against laboratory and clinical isolates of HIV-1 was assessed in T lymphoblastoid cell lines, monocyte/macrophage cells, and primary peripheral blood lymphocytes. The 50% effective concentrations (EC50) ranged from 0.02 to 1.7 nM. Elvitegravir displayed antiviral activity in cell culture against HIV-1 clades A, B, C, D, E, F, G, and O (EC50 values ranged from 0.1 to 1.3 nM) and activity against HIV-2 (EC50 value of 0.53 nM). Elvitegravir did not show inhibition of replication of HBV or HCV in cell culture.

Cobicistat: Cobicistat has no detectable antiviral activity in cell culture against HIV-1, HBV, or HCV and does not antagonize the antiviral activity of elvitegravir, emtricitabine, or tenofovir.

Emtricitabine: The antiviral activity of emtricitabine against laboratory and clinical isolates of HIV-1 was assessed in T lymphoblastoid cell lines, the MAGI-CCR5 cell line, and primary peripheral blood mononuclear cells. The EC50 values for emtricitabine were in the range of 0.0013–0.64 micromolar. Emtricitabine displayed antiviral activity in cell culture against HIV-1 clades A, B, C, D, E, F, and G (EC50 values ranged from 0.007–0.075 micromolar) and showed strain-specific activity against HIV-2 (EC50 values ranged from 0.007–1.5 micromolar).

Tenofovir DF: The antiviral activity of tenofovir against laboratory and clinical isolates of HIV-1 was assessed in T lymphoblastoid cell lines, primary monocyte/macrophage cells and peripheral blood lymphocytes. The EC50 values for tenofovir were in the range of 0.04–8.5 micromolar. Tenofovir displayed antiviral activity in cell culture against HIV-1 clades A, B, C, D, E, F, G, and O (EC50 values ranged from 0.5–2.2 micromolar) and showed strain-specific activity against HIV-2 (EC50 values ranged from 1.6–5.5 micromolar).

Resistance

In Cell Culture

Elvitegravir: HIV-1 isolates with reduced susceptibility to elvitegravir have been selected in cell culture. Reduced susceptibility to elvitegravir was associated with the primary integrase substitutions T66A/I, E92G/Q, S147G, and Q148R. Additional integrase substitutions observed in cell-culture selection included D10E, S17N, H51Y, F121Y, S153F/Y, E157Q, D232N, R263K, and V281M.

Emtricitabine and TDF: HIV-1 isolates with reduced susceptibility to emtricitabine or tenofovir have been selected in cell culture. Reduced susceptibility to emtricitabine was associated with M184V/I substitutions in HIV-1 RT. HIV-1 isolates selected by tenofovir expressed a K65R substitution in HIV-1 RT and showed a 2–4 fold reduction in susceptibility to tenofovir.

In Clinical Studies

Elvitegravir: Development of substitutions T66A/I/K, E92G/Q, T97A, S147G, Q148H/K/R, and N155H in the HIV-1 integrase protein was primarily associated with resistance to elvitegravir. In addition to these primary elvitegravir resistance-associated substitutions, E92A, F121C/Y, P145S, Q146I/L/R, and N155S were also occasionally observed and were shown to confer reduced susceptibility to elvitegravir. In virus isolates harboring the observed primary elvitegravir resistance-associated substitutions, additional substitutions in integrase were detected including H51Y, L68I/V, G70R, V72A/N, I73V, Q95K/R, S119R, E138A/K, G140A/C/S, E157Q, K160N, E170A, S230R, and D232N.

Emtricitabine and TDF: HIV-1 isolates with reduced susceptibility to emtricitabine or tenofovir have been selected in subjects experiencing virologic failure in clinical trials. Genotypic analysis of these isolates identified the M184V/I and K65R amino acid substitutions in the viral RT, respectively.

Elvitegravir, Cobicistat, Emtricitabine, and TDF: In clinical trials of HIV-1-infected subjects with no antiretroviral treatment history, Studies 102 and 103 [see Clinical Studies (14)], by Week 144, the development of one or more primary substitutions associated with resistance to elvitegravir, emtricitabine, and/or tenofovir was observed in viruses from 51% (18/35) of the STRIBILD-treatment failure subjects with evaluable genotypic resistance data who received at least 8 weeks of STRIBILD and had HIV-1 RNA greater than or equal to 400 copies per mL at confirmed virologic failure, the end of each study year, or the time of early study drug discontinuation. The most common substitutions that emerged were M184V/I (N=17) in HIV-1 RT and the primary elvitegravir resistance-associated substitutions, E92Q (N=9), N155H (N=5), Q148R (N=3), T66I (N=2), and T97A (N=1) in integrase; K65R in RT was also detected (N=5). In virus isolates harboring the observed primary elvitegravir resistance substitutions, additional substitutions in integrase were detected including H51Y, L68I/V, G70R, I73V, G140C, S153A, E157Q, and G163R. The virus in all subjects with evaluable data for RT and IN and whose virus developed integrase substitutions associated with elvitegravir resistance (N=14) also developed the M184I/V RT substitutions, and had reduced susceptibility to both elvitegravir and emtricitabine. In phenotypic analyses, HIV-1 isolates expressing M184V/I RT substitutions showed reduced susceptibility to emtricitabine (42- to greater than 152-fold); those expressing the primary elvitegravir resistance-associated integrase substitutions showed reduced susceptibility to elvitegravir (4- to greater than 198-fold); and those expressing the K65R RT substitution showed reduced susceptibility to tenofovir (0.8- to 1.6-fold), compared to wild-type reference HIV-1.

There was an insufficient number of virologic failures with evaluable data (N=1) in clinical trials of virologically suppressed HIV-1-infected subjects with no history of virologic failure, studies 115 and 121, [see Clinical Studies (14)] to draw conclusions about the development of resistance.

Cross-Resistance

STRIBILD-treatment failure subject isolates exhibited varying degrees of cross-resistance within the INSTI and NRTI drug classes depending on the specific substitutions observed. These isolates remained susceptible to all NNRTIs and protease inhibitors.

Elvitegravir: Cross-resistance has been observed among INSTIs. Elvitegravir-resistant viruses showed varying degrees of cross-resistance in cell culture to raltegravir depending on the type and number of substitutions in HIV-1 integrase. Of the primary elvitegravir resistance-associated substitutions tested (T66A/I/K, E92G/Q, T97A, S147G, Q148H/K/R, and N155H), all but three (T66I, E92G, and S147G) conferred greater than 1.5-fold reduced susceptibility to raltegravir (above the biological cutoff for raltegravir) when introduced individually into a wild-type virus by site-directed mutagenesis. Of the primary raltegravir resistance-associated substitutions (Y143C/H/R, Q148H/K/R, and N155H), all but Y143C/H conferred greater than 2.5-fold reductions in susceptibility to elvitegravir (above the biological cutoff for elvitegravir).

Emtricitabine: Cross-resistance has been observed among NRTIs. Emtricitabine-resistant isolates harboring an M184V/I substitution in HIV-1 RT were cross-resistant to lamivudine. HIV-1 isolates containing the K65R RT substitution, selected in vivo by abacavir, didanosine, and tenofovir, demonstrated reduced susceptibility to inhibition by emtricitabine.

Tenofovir DF: Cross-resistance has been observed among NRTIs. The K65R substitution in HIV-1 RT selected by tenofovir is also selected in some HIV-1-infected patients treated with abacavir or didanosine. HIV-1 isolates with the K65R substitution also showed reduced susceptibility to emtricitabine and lamivudine. Therefore, cross-resistance among these NRTIs may occur in patients whose virus harbors the K65R substitution. The K70E substitution selected clinically by TDF results in reduced susceptibility to abacavir, didanosine, emtricitabine, lamivudine, and tenofovir. HIV-1 isolates from patients (N=20) whose HIV-1 expressed a mean of 3 zidovudine-associated RT amino acid substitutions (M41L, D67N, K70R, L210W, T215Y/F, or K219Q/E/N) showed a 3.1-fold decrease in the susceptibility to tenofovir. Subjects whose virus expressed an L74V RT substitution without zidovudine resistance-associated substitutions (N=8) had reduced response to TDF. Limited data are available for patients whose virus expressed a Y115F substitution (N=3), Q151M substitution (N=2), or T69 insertion (N=4) in HIV-1 RT, all of whom had a reduced response in clinical trials.